Sunetra Gupta, professor of theoretical epidemiology at Oxford, led a study into the infection rate of Covid-19 across the country.
The findings came on the same day the official death toll jumped to a record 87 in one day to 422 and confirmed cases leapt by 1,427.
The new model from Oxford University suggests the virus was circulating in the UK by mid-January, around two weeks before the first reported case and a month before the first reported death.
And the research suggested that less than one thousand of with Covid-19 became ill enough to need treatment in hospital, with the vast majority developing mild symptoms or none at all.
This means it could have had enough time to have spread widely, with many people in the country acquiring immunity.
Speaking to the Financial Times, Professor Gupta said testing was needed to assess the theory.
Oxford’s research represents a very different view to the modelling at Imperial College London, which influenced government policy to tackle the spread of the virus in the UK.
“I am surprised that there has been such unqualified acceptance of the Imperial model,” Professor Gupta told the same newspaper.
But she was reluctant to criticise Boris Johnson’s decision to place the country on lockdown as the accuracy of the new model has not yet been confirmed.
If accurate, the results would mean the country has already acquired substantial “herd immunity” through the unrecognised spread of the illness.
Herd immunity is the idea that coronavirus will stop spreading when enough people have become resistant after being infected.
The shutdown across the UK could be removed much sooner expected if the findings are confirmed by testing, it was reported.
It is hoped testing will begin as soon as this week.
More than 90,000 people have been tested for Covid-19 in the UK, 82,359 of which came back negative.
NHS England said 83 of the 87 new deaths in the UK happened in England, bringing the total number of deaths there to 386.
Patients were aged between 33 and 103 years old and all were in vulnerable groups including those with underlying conditions.
According to NHS England, 21 of the deaths were reported at London North West University Healthcare NHS Trust in Harrow.
A further seven died at the Princess Alexandra Hospital in Harlow.
Four died at each of Manchester University NHS Foundation Trust, University Hospitals of Leicester and Leeds Teaching Hospitals NHS Trust.
The latest figures were announced as the country enters its first day of lockdown after Boris Johnson announced people should only leave their homes for four reasons.
People can leave their homes to shop for essentials, for one form of exercise per day, for medical need or to provide care to a vulnerable person and travelling to and from essential work.