Between 9,000 and 16,000 Britons are getting infected with coronavirus every day, according to researchers monitoring the UK's outbreak.
King's College London (KCL) scientists behind the COVID Symptom Tracker mobile app estimate that there were at least 16,310 daily cases of the disease in the last week.
The Office for National Statistics, a Government-run agency, has made a more modest estimate today, saying it thinks around 9,600 people are contracting the virus every day.
Both surveillance projects are picking up far more than the Government's official testing programme, which recorded 6,000-plus cases on Thursday and Wednesday.
KCL collects its data by sending tests to people who report tell-tale symptoms of Covid-19 into the mobile app, while the ONS study sends tests to random households regardless of their health status.
The university has based its latest estimates on more than 6,000 tests this week, of which 151 were positive - about three times more than the ONS'.
Data from the symptom-tracking app, which has seen millions of Brits sign up and report their symptoms, suggests there are nearly 150,000 people currently suffering symptomatic Covid-19, although many more will have no symptoms.
This figure has more than doubled since last week, when there were about 70,000 symptomatic patients. The chief scientist behind the app said it was fresh evidence the crisis was 'rising at an alarming rate'.
Almost half of the new daily infections are occurring in the North of England (7,778) but London, Glasgow and Belfast are also seeing 'worrying' rises, according to the KCL team.
Both surveillance projects are picking up far more than the Government's official testing programme, which recorded 6,000-plus cases on Thursday and Wednesday
Data from the app, which has seen millions of Brits sign up and report their symptoms, suggests there are nearly 150,000 people currently suffering symptomatic Covid-19, although many more will have no symptoms
In London, infections have more than doubled from 9,291 to 18,200, the equivalent of 213 new cases per million population as of September 20
The fresh batch of data was based on 6,847 swab tests done between September 7 and September 20 from people right across the UK, during which 151 people tested positive for the virus.
KCL researchers then extrapolate this data to the general population to make estimates about the virus's trajectory. Their highly accurate data is being fed into Government to help steer it through the crisis.
The app estimates 147,498 people have symptomatic Covid-19 in the UK right now, with 55,201 patients in England, 14,319 in Scotland and 9,075 in Wales. They did not make estimates for Nothern Ireland.
Broken down, the North of England is being battered hardest by the latest surge in infections, but London, Glasgow and Belfast are also seeing 'worrying' rises, according to the KCL team
Broken down, the North West of England is being battered hardest by the latest surge in infections, with estimated cases tripling in the last seven days from 12,544 to 36,316.
In the North East and Yorkshire and London, infections have more than doubled from 12,916 to 27,731 and 9,291 to 18,200, respectively.
The researchers now predict the reproduction 'R' rate - the average number of people each Covid-19 patient infects - is dangerously high across the UK - 1.4 in England and Wales and 1.3 in Scotland.
Experts say keeping the R squaashed below 1.0 is essential to prevent the outbreak from growing exponentially and spiralling out of control.
Tim Spector, professor of genetic epidemiology at King's College London and the brains behind the app, said: 'The number of cases in the UK continues to rise at an alarming rate as we are seeing figures doubling weekly across the country, in particular we are worried about places like London and other major cities like Manchester, Belfast and Glasgow where cases are surging and the R value is around 1.4.'
Yesterday, Britain recorded 6,634 more cases of coronavirus in the highest daily toll on record.
It took the official infection toll to 416,363, although millions of Brits went undiagnosed during the first wave of the pandemic due the government's lacklustre testing regime. Top experts say more than 100,000 people were actually catching the virus every day during the darkest days of the first wave.
The 6,634 new infections in the last 24 hours mark a twofold rise compared to last Thursday, when 3,395 people were diagnosed with the disease. Yesterday saw another 6,178 infections recorded. Data shows the rolling seven-day average of daily cases has jumped 48 per cent in a week.
Infections were squashed well below 1,000 from late June until early August following the lockdown in spring, but Covid-19 cases have been on the rise ever since.