The UK's critical coronavirus R number has risen above 1.3, new data shows.
In England it is believed to be 1.4, while in Scotland and Wales it is 1.3 - meaning case numbers are doubling every seven days, according to a symptom tracker app.
It is estimated that 69,686 people currently have symptomatic Covid-19 - nearly double last week's figure of 35,248.
Last week, the UK Government Sage committee's official estimate for R across the UK was between 1.0 and 1.2. Before then, the last time R was above 1 was in early March.
R is the number of people each infected person passes the virus on to, on average, and it needs to be kept below one in order for Covid-19 to stay in control.
Meanwhile, a second national lockdown to curb the spread of coronavirus has not been ruled out, the Health Secretary has said, as the number of hospital admissions doubles every eight days.
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Matt Hancock said a national lockdown was the "last line of defence" as he responded to reports that ministers are considering further national measures, even for just a two-week period, such as imposing a curfew on bars and restaurants.
It comes as ministers come under fire over the NHS Test and Trace system, which has seen up to four times the number of people trying to book a test as the number of tests available.
Experts have said that without effective testing and tracing, it will be much harder to control the spread of the virus and pinpoint larger outbreaks.
Mr Hancock told BBC Breakfast the latest data showed that hospital admissions are now doubling every eight days, amid warnings that deaths will rise in the coming weeks.
He said it was "absolutely critical" that people followed the rule of having no more than six people at a gathering, while those living under local restrictions should ensure they are sticking to advice.
"Also, if people have tested positive, or if people have been in close contact with somebody who tests positive, that they self-isolate," he said.
"And if we do all these things, then we can avoid having to take serious further measures."
He said the current approach was "targeted interventions" but "a national lockdown is the last line of defence".
He added: "As we saw in the spring, it is the thing that we can do to keep people safe if that's needed.
"So we're watching vigilantly, but we can see the number of cases accelerating, and we're prepared to do what it takes both to protect lives and to protect livelihoods, and of course, both are so important.
"We want to avoid a national lockdown but we're prepared to do it, if we need to."
Mr Hancock said the "big hope" is that people will come together "and get this under control, but it is a very serious situation."
Asked on Sky News about the possibility of a two-week "circuit break" imposition of national restrictions, Mr Hancock said the Government wanted to use "local action".
And he told BBC Radio 4's Today programme: "We want to avoid national lockdown altogether.
"I have learned over the last nine months not ever to rule anything out.
"However, it is not the proposal that's on the table."
Scientists from the Government's Scientific Advisory Group for Emergencies (Sage) have reportedly proposed a two-week national lockdown in October to tackle the rising number of Covid-19 cases.
The Financial Times reported that they had said a lockdown could coincide with the October school half-term.
Earlier this week, Prime Minister Boris Johnson described the potential impact of a second national lockdown on the economy as "disastrous".
At the moment, more than 10 million people across the UK are living under local restrictions, covering parts of Scotland, South Wales, the North West and North East of England, Yorkshire and the Midlands.
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Mr Hancock said further local action would be announced later amid reports that tougher restrictions are expected to be imposed on Lancashire and Merseyside.
It comes as the head of NHS Test and Trace denied it was "failing", despite a dramatic slowdown in turnaround times for tests and scenes of people turning up at A&E trying to get a test.
Baroness Dido Harding - head of NHS Test and Trace - acknowledged demand was significantly outstripping capacity.
But she said the size of the system had been based on modelling by the Government's scientific advisers, and suggested the problems were exacerbated by people without symptoms seeking tests for which they were ineligible.
New testing figures for England showed 33.3% of people who were swabbed at a regional site, local site or mobile testing unit received their result within 24 hours - despite Mr Johnson's promise that they would all be turned around within that timescale by the end of June.
The latest figure showed an ability to carry out 242,817 tests a day, but the Government has pledged that will increase to 500,000 by the end of October.
Meanwhile, the Government is expected to announce tighter restrictions on care home visits in areas with high numbers of coronavirus cases.
Care homes in areas subject to local lockdowns may be advised to temporarily restrict visits in all but end-of-life situations, it is understood.
For parts of the country where there is no local lockdown, but where community transmission is a cause for concern, an option officials are considering is advising that visits are restricted to one designated visitor per resident.
The Government will set out further details on Friday in its social care action plan to help fight the spread of coronavirus over winter.
Figures on Thursday showed there had been a further 3,395 lab-confirmed cases of coronavirus in the UK, and 21 more people had died within 28 days of testing positive.
Cases have been doubling every eight days, according to official data.
Elsewhere, Thailand and Singapore have been added to the list of travel corridors, meaning travellers arriving in the UK from those countries after 4am on Saturday will no longer need to self-isolate for 14 days, but Slovenia and Guadeloupe have been removed.