The Prime Minister made the statement after he was asked whether France would be removed from the Government's safe-travel list. Britain has in recent weeks imposed a 14-day quarantine period for arrivals from countries like Spain and Belgium, responding to rising infections and fears of a second wave of the virus, having initially declared them safe for travel.
Speaking during a visit to Northern Ireland, Mr Johnson warned ministers would review the rules for France tonight.
He said: "We've got to be absolutely ruthless about this, even with our closest and dearest friends and partners around the world.
"We will be looking at the data a bit later on this afternoon ... looking at exactly where France and other countries are getting to, and you know we can't be remotely complacent about our own situation."
France recorded 2,669 new cases of coronavirus on Thursday - an increase on the 2,524 declared Wednesday,
Steve Swinford, deputy political editor of The Times, warned quarantine rules for British travellers in France are "almost inevitable".
He said: "Expect a 30 hour notice period if quarantine does come in."
The UK’s coronavirus death toll has been reduced but 1,000 new cases were confirmed on Wednesday
The warning comes as the UK's coronavirus death toll was reduced by more than 5,000 after changes to the way figures are calculated - but in a grim update the number of daily new cases has soared above 1,000 again, sparking fresh fears of a second wave.
A further 1,009 coronavirus infections were reported on Wednesday, taking the total number of confirmed cases to 313,798. The jump has stoked fears of a second peak of COVID-19 in Britain as multiple European countries also grapple with rising infection rates.
The new cases come as a review of how coronavirus deaths are calculated in England has reduced the UK’s death toll by more than 5,000 to 41,329.
Previously, figures from the Department of Health included anyone who had died having tested positive for the disease - regardless of whether it played a factor in their death.
From now on, only deaths that occur within 28 days of a postive test will be included in the daily figures.
FOLLOW BELOW FOR LIVE UPDATES:
France has seen a spike in coronavirus cases in recent days
12.40am update: Brazil's COVID-19 death toll exceeds 105,000
Brazil reported 60,091 new COVID-19 infections and 1,261 deaths from coronavirus in the past 24 hours.
This brings the total number of deaths related to COVID-19 to 105,463 according to Brazil’s health ministry.
The country has reported 3,224,876 cases of coronavirus since the pandemic started.
Brazil is the second worst country hit by the coronavirus after the US.
11.45pm update: Lockdown measures in UK to ease further in England
Additional beauty treatments, small wedding receptions and live indoor performances will be able to take place in England from Saturday.
Prime Minister Boris Johnson announced that casinos, soft play centres and bowling alleys will also be able to reopen.
Mr Johnson said the plans “will allow more people to return to work and the public to get back to more of the things they have missed”.
But he warmed that the Government "will not hesitate to put on the brakes if required, or to continue to implement local measures to help to control the spread of the virus".
Manon Dark takes over from Laura O'Callaghan
10.37pm update: Top US health official says approval of COVID vaccines unlikely before November
Any potential COVID-19 vaccine backed by the Trump administration's "Operation Warp Speed" program is unlikely to receive a green light from regulators any earlier than November or December, given the time needed for a large-scale clinical trial, the National Institutes of Health director said on Thursday.
In a call with reporters, Francis Collins said he thinks testing a vaccine in at least 10,000 people could potentially give enough evidence of safety and efficacy to clear it for wider use.
US late-stage vaccine trials launched so far aim to recruit up to 30,000 people.
Dr Collins said: "I would not expect to see, on the basis of what we know scientifically, that we would be at the point where the FDA could make such a judgment until considerably later than October 1st."
He was referring to the US Food and Drug Administration.
He added: "Maybe November or December would be my best bet."
He added that he is confident that at least one of the six vaccines funded by the initiative will be shown to be safe and effective by the end of the year.
9.56pm update: France, Netherlands and Malta added to UK's quarantine list
Travellers returning from the three countries will be required to self-isolate for 14 days.
9.32pm update: Coventry COVID outbreak: Nearly 200 banana packers to be tested after staff contract virus
Nearly 200 staff members at a fruit distribution centre are to be urgently tested for coronavirus after 10 workers contracted the disease.
Bosses at Fyffes Group Ltd in Coventry have taken drastic action in a bid to quell an outbreak of the potentially deadly virus.
All 186 staff will undergo testing as a "precaution", Coventry City Council said. Social distancing measures at the plant will also be stepped up.
For the full story click here.
A top US health official says a Covid vaccine is unlikely before November
8.37pm update: Trump holds up coronavirus aid to block Democrats' bid for election funding
President Donald Trump on Thursday said his team was rejecting Democrats' bid to include funds for the US Postal Service and shore up election infrastructure in a new coronavirus relief bill, as he vowed to block money to facilitate mail-in voting.
Congressional Democrats accused Mr Trump of trying to damage the struggling Postal Service in an effort to improve his chances of being re-elected in November.
The Republican president has been railing against mail-in ballots for months as a possible source of fraud, although millions of Americans - including much of the military - have used the post office to cast absentee ballots for years without such problems.
Mr Trump said his negotiators have resisted Democrats' calls for additional money to help prepare for presidential, congressional and local voting during a pandemic that has killed more than 165,000 Americans and presented severe logistical challenges to organizing large events like the November 3 election.
7.29pm update: Warning over virus 'spreading rapidly' on Orkney island
Five positive coronavirus cases have been confirmed across Orkney as the head of the local health board warned the virus is "spreading rapidly" on the islands.
The cluster is linked to a fishing vessel with an investigation now under way alongside contact tracing across NHS Highland, NHS Grampian and NHS Orkney.
Earlier on Thursday, NHS Orkney chief executive Michael Dickson said a number of people had developed symptoms and then travelled to their homes across Orkney and to mainland Scotland.
He said the cluster involves a minimum of nine people, some of whom have tested positive, with work ongoing to trace contacts.
6.45pm update: UK death toll rises by 18
The UK's death toll rose by 18 on Thursday.
New rules to fatality numbers mean only patients who died within 28 days of receiving a positive test result were accounted for.
The death toll now stands at 41,347.
Separate figures published by the UK's statistics agencies show there have been 56,800 deaths registered in the UK where COVID-19 was mentioned on the death certificate.
The Government said: "We have not received the latest data for cases and tests in England. We will update today's records as soon as they become available."
Boris Johnson said ministers would review quarantine rules for France on Thursday night
6.41pm update: AstraZeneca set to start making 400 million COVID-19 vaccines for Latam early in 2021
Production of 400 million doses of a COVID-19 vaccine for Latin America could begin early next year, an executive for pharmaceutical company AstraZeneca Plc said on Thursday, as the region's coronavirus death toll stands at nearly 230,000.
In partnership with the Mexican and Argentinean governments, AstraZeneca plans to initially produce 150 million doses, and eventually make at least 400 million for distribution throughout the region, said Sylvia Varela, head of AstraZeneca Mexico.
Home to some 650 million people, Latin America has registered the world's highest tallies for coronavirus cases and deaths, with Brazil and Mexico trailing only the United States in record numbers of fatalities.
Ms Varela said: "We'll be prioritising the vulnerable populations.
She noted that the pricing, while still not final, was not expected to exceed $4 per dose.
That could bring the cost of the first 150 million doses to $600 million.
5.30pm update: WHO downplays danger of coronavirus entering food chain
The World Health Organisation on Thursday downplayed the danger of the coronavirus latching on to food packaging and urged people not to be afraid of the virus entering the food chain.
The WHO's head of emergencies programme Mike Ryan said: "People should not fear food, food packaging or delivery of food.
"There is no evidence the food chain is participating in transmission of this virus."
4.11pm update: Government will be 'ruthless' with strict quarantine measures, says PM
Boris Johnson has said the Government will be "absolutely ruthless" in deciding whether to impose a requirement on people arriving in the UK from France to quarantine for 14 days.
The Prime Minister said officials would be looking at the latest coronavirus figures later on Thursday.
He told reporters during a visit to Northern Ireland: "We have got to be absolutely ruthless about this, even with our closest and dearest friends and partners. I think everybody understands that.
"We will be looking at the data a bit later on this afternoon - looking exactly where France and other countries are getting to.
"We can't be remotely complacent about our own situation. Everybody understands that in a pandemic you don't allow our population to be reinfected or the disease to come back in.
"That is why the quarantine measures are very important and we have to apply them in very strict way."
3.50pm update: No new fatalities in Northern Ireland
No new deaths with Covid-19 have been reported in Northern Ireland, the Department of Health said on Thursday.
An additional eight people tested positive.
3.10pm update: Greater Manchester Mayor warns against 'knee-jerk measures' over Covid
Greater Manchester Mayor Andy Burnham has called on the Government not to bring in "knee-jerk measures" as coronavirus cases continue to rise in some boroughs.
Mr Burnham said he did not want to see a full lockdown in Oldham, where the rate of infections is the highest in the country.
But he said that he and Manchester City Council leader Sir Richard Leese had written to Health Secretary Matt Hancock to ask that there was no further easing of restrictions in the city region this weekend.
Extra restrictions on socialising between households were introduced in Greater Manchester, as well as parts of Lancashire and Yorkshire, at the end of July following a spike in cases.
2.50pm update: England hospital death toll rises by 13
A further 13 people who tested positive for coronavirus have died in hospital in England.
It brings the total number of coronavirus deaths in hospitals in England to 29,444.
2.30pm update: Coronavirus cases in Wales up 15
The number of coronavirus cases in Wales has risen by 15 to 17,499.
There have been no new coronavirus deaths with the toll at 1,586.
2.10pm update: Coronavirus outbreak confirmed at sandwich factory
A coronavirus outbreak has been confirmed at a sandwich factory in Northampton.
Greencore said some of its staff are self-isolating after testing positive for coronavirus.
In a statement, the company said: "As a result of the rising number of Covid-19 cases in the Northampton area, we took the decision to start proactively testing all of the colleagues at our Northampton site.
"We can confirm that a number of colleagues have tested positive for the virus and are now self-isolating.
"We are liaising closely with PHE East Midlands, Northamptonshire County Council and Northampton Borough Council, who are fully supportive of the controls that we have on site."
1.25pm update: Sturgeon says coronavirus clusters are inevitable and no cause for undue alarm
A number of new coronavirus clusters in Scotland should not cause undue alarm, First Minister Nicola Sturgeon has said.
Clusters have emerged in Glasgow and Orkney in addition to the one in Aberdeen which led to a local lockdown.
Speaking at the Scottish Government’s daily briefing, the SNP leader said "clusters of this kind, I'm afraid to say, are inevitable”.
She added: "We should become more used to hearing about several different clusters because the virus hasn't gone away.
"While we shouldn't be unduly alarmed, it is equally important that we're not in any way complacent."
12.50pm update: Scotland coronavirus cases increase by 47
Scotland has recorded 47 new coronavirus cases in the last 24 hours with the total at 19,173, First Minister Nicola Sturgeon said during the Scottish Government’s daily briefing.
There have been no further deaths with the toll remaining at 2,491.
A number of new coronavirus clusters in Scotland should not cause undue alarm, First Minister Nicola Sturgeon has said
12.15pm update: Total of 52,735 cases transferred to NHS Test and Trace
A total of 52,735 people who tested positive for coronavirus in England have had their cases transferred to the NHS Test and Trace contact tracing system since its launch, according to figures from the Department of Health and Social Care.
Of this total, 41,254 people (78.2 percent) were reached and asked to provide details of recent close contacts, while 9,938 (18.8 percent) were not reached.
A further 1,543 people (2.9 percent) could not be reached because their communication details had not been provided.
The figures cover the period May 28 to August 5.
12pm update: Vietnam reports 25 more coronavirus cases
Vietnam has recorded 25 further coronavirus infections and three new deaths.
It brings the total number of cases to 905 and the death toll to 20.
More than 430 of the total cases are linked to an outbreak in the city of Danang which started last month.
11.10am update: Indonesia records 2,098 new coronavirus infections
Indonesia has reported 2,098 new coronavirus cases with the total reaching 132,816.
The country’s death toll increased by 65 to 5,968.
Vietnam has recorded 25 further coronavirus infections and three new deaths
11.40am update: New contract tracing app trial to start
Residents of the Isle of Wight, the London borough Newham, and NHS volunteer responders across England will be part of a new contact tracing app trial, the Department for Health and Social Care has said.
The app, which uses Google and Apple-developed technology, will include alerts based on postcode, QR check-in at venues, a symptom checker and a way to book tests.
Trials will start from Thursday on the Isle of Wight and with NHS volunteer responders across England, followed shortly by Newham residents, as part of the country's NHS Test and Trace service.
It will log the time and distance a user has spent near to anyone, even if they don't know them, so it can alert them if that person later tests positive for coronavirus.
10.55am update: Hong Kong coronavirus cases up by 69
Hong Kong has recorded 69 further coronavirus cases of which 65 were locally transmitted.
More than 4,200 people have been infected in Hong Kong and 65 have died.
Hong Kong has seen a resurgence in cases since early July.
Gavin Williamson has ruled out making further changes to exam grades after the Government made last-minute alterations to the system
10am update: Russia reports 5,057 further coronavirus cases
Russia has confirmed 5,057 new coronavirus cases taking the total number of infections to 907,758.
The country has recorded 124 further deaths bringing the toll to 15,384.
9.20am update: Education Secretary rules out further changes to exam grading process
Gavin Williamson has ruled out making further changes to exam grades after the Government made last-minute alterations to the system.
Students in England are receiving calculated grades after this summer's A-level and GCSE exams were cancelled due to the Covid-19 pandemic.
The Government announced late on Tuesday that youngsters can use mock exam results as the basis for an appeal if they are higher than the calculated grade.
It came after protests from pupils in Scotland prompted the SNP to allow results estimated by teachers to be accepted.
Asked if he was prepared to change the system again, Mr Williamson told Times Radio: "We're not going to be changing this system again.
"We believe that we've put in place - in terms of the triple lock, in terms of the actions we've taken - a system that is able to put its arm round those youngsters where there has been a grade that has been unfair on them and is able to put that right."
Some 3.4 million people in England have been infected with coronavirus, a new study suggests
8.50am update: Ukraine records record daily high of coronavirus cases
Ukraine has recorded 1,592 new coronavirus cases - a record daily increase.
The number of infection has risen in the country over the past two months as some lockdown measures have been eased.
Health minister Maksym Stepanov urged people to obey broader restrictions that remain in place.
He said: "I will insist on strict adherence to all the rules. The situation is very tense.”
The number of confirmed cases rose to 86,140 and the death toll is 1,992.
8.15am update: 3.4m people in England have had coronavirus, study suggests
Some 3.4 million people in England have been infected with coronavirus, a new study suggests.
The study involved more than 100,000 volunteers using a finger-prick home test to find antibodies for the virus and was carried out by Imperial College London.
The research suggested 6 percent of England's population had already been infected with coronavirus by July 13.
UK coronavirus map as of August 12
7.50am update: India coronavirus cases jump by 67,000 in new daily record
India has reported 66,999 further coronavirus cases in a record daily rise taking the total number of confirmed infections to 2.4 million, the country’s health ministry said.
The death toll from the virus has passed 47,000.
7.25am update: UK coronavirus outbreak ‘widely dispersed’ from start
The coronavirus outbreak in the UK appears to have been "widely dispersed" from the start, one of the scientists leading a study into the prevalence of the deadly virus has said.
The first patient to die with coronavirus in Britain was recorded on March 5, when it was thought there were only 90 cases in the UK.
Helen Ward, from Imperial College London, told BBC Breakfast: "What was interesting (about the study) is that we can tell from people who reported not only having a positive test, but we also asked about their symptoms so we can actually track for most people - the 70 percent of people who reported symptoms - when they think they were infected.
"And it did start in January, February, and actually it started right across the country, so you can't say that it started in London and spread out. Right from the beginning it was widely dispersed."