Data suggests daily Covid cases in the UK may have reached almost 100,000 - double the official number.
The Zoe Covid Study recorded an average of 92,953 daily infections over five days, compared with the 44,702 fresh cases announced by the Government on Friday.
The Zoe figures are based on both PCR and lateral flow tests, with the number of cases rising by 14% on the 81,823 new infections recorded the week before.
The data suggests one in 56 people in the UK currently have symptomatic Covid.
Professor Tim Spector, the top scientist on the Zoe Study, said the app’s statistics are consistently higher than official figures because they include self-reported lateral flow tests.
It comes after Health Secretary Sajid Javid warned that Covid infections could hit 100,000 a day, with the prospect of restrictions being reintroduced this winter.
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Prof Spector said: “The Zoe data shows the UK could hit 100,000 new cases sooner than expected and with no sign of a Plan B or Plan C.
“The government raw figures report on PCR testing of the classical symptoms only, which miss around 40% of cases.
“It’s clear the government figures are a big under-estimate, and with the highest rates in Western Europe, there’s no room for complacency.
“With the UK government delaying any decision while hospitals fill up, it’s time to take matters into our own hands to address the worsening situation.
“Public action has worked in previous waves to reduce rates before lockdowns.”
The Zoe data suggests infections are rising among all age groups, although under 18s have recorded the highest proportion of cases.
South-west England has seen a particularly sharp spike in infections, according to the app.
The app’s figures are based on reports from around 750,000 contributors and the number of people who have tested positive for Covid.
Prof Spector said data showed vaccine protection is waning and he urged those eligible for a booster jab to take up the offer.
He also encouraged people to wear masks on public transport and in crowded places.
It comes after reports that a ban on household mixing could make a return with Brits urged to take Covid tests before Christmas gatherings with family and friends.
Cabinet Office officials are said to be discussing proposals for a clamp down on socialising, which could form part of a “Plan C” if booster jabs don't bring the virus under control by Christmas.
Health Minister Edward Argar said it wasn't "being actively considered" but the Government would always look at ways to prevent the virus spiralling out of control.
The Government drew up a list of "Plan B" contingency measures as part of efforts to fight Covid this winter, which includes a return to mandatory face masks, working from home and vaccine passports.
But despite rising cases, Mr Javid has resisted calls to implement the measures.
