Scientists tracking the spread of coronavirus in England say infection levels in the community may have risen at the start of the latest lockdown.
Infections in 6-15 January were up by 50% on early December, with one in 63 people infected, Imperial College London's initial findings suggest.
Swab tests from 143,000 people indicate 1.58% had the virus during in early January - up from 0.91% in December.
Ministers say the report does not yet reflect the impact of the lockdown.
The latest round of results from Imperial College's React-1 infection survey - one of the country's largest studies into Covid-19 infections - are interim with the full set of results to be published in a weeks' time.
But Imperial College London's Prof Paul Elliott warned if the high prevalence continues "more lives will be lost".
The report also says there are "worrying suggestions of a recent uptick in infections" and Prof Elliott said the third lockdown - introduced on 6 January - was not having the same impact as the first, in April.
London had the highest level in the January period - 2.8%, up from 1.21% in early December.