Covid-19 infections may have risen during the first week of the national lockdown, experts have warned, sparking concern over the effectiveness of the current restrictions.
The prevalence of coronavirus across England increased by 50 per cent between early December and the second week of January, according to the latest React study from Imperial College London and Ipsos Mori.
More than 142,900 volunteers were tested between 6 and 15 January, revealing that one in 63 people had Covid-19.
The report, which researchers said does not yet reflect the full impact of the UK-wide lockdown, also showed there were "worrying suggestions of a recent uptick in infections".
The study authors said the national R – referring to the number of people that an infected person will pass the virus on to – was estimated to be 1.04.
Professor Paul Elliot, who is leading the React study, suggested the current restrictions may not be strict enough to see a drop in infections and the R rate.
"We're in a position where the levels are high and are not falling now within the period of this current lockdown,” he told BBC Radio 4's Today programme.
"We're seeing this levelling off, it's not going up, but we're not seeing the decline that we really need to see given the pressure on the NHS from the current very high levels of the virus in the population."
Steven Riley, professor of infectious disease dynamics at Imperial, said the study had examined a long enough time period to assess the current lockdown.
"It's long enough that, were the lockdown working effectively, we would certainly have hoped to have seen a decline," he said.
He said data from previous lockdowns did show a fall, adding that the current research "certainly doesn't support the conclusion that lockdown is working".
On what he expects would happen in the current lockdown, he said: "We would expect a similar plateau, a very gradual increase (of infections), if behaviour stays the same and, if our interpretation is correct, if what we are seeing is kind of the result of the post-Christmas period behaviour."
The react study showed that while there was a rise in prevalence across all adult age groups, it was highest in 18 to 24 year olds, and more than doubled in the over 65s age group.
London saw the highest regional prevalence, jumping from 1.21 per cent to 2.8 per cent, while there were also rises in the southeast, east of England, West Midlands, southwest and northwest.
The only region to see a decrease was Yorkshire and the Humber, and prevalence remained stable in the East Midlands and north east, but the researchers warned infection numbers are still high even in these areas.
Government data shows that the number of new cases of Covid-19 per head of population has been falling in all regions of England in recent days.
The government said the React study does not yet take full account of the current lockdown measures.
For the first time the report has included mobility data, showing peoples' movement decreased at the end of December and increased at the start of January, which the scientists said helps to explain the change in prevalence.
Prof Elliott warned that if the high rates continue to persist "more and more lives will be lost".
"To prevent our already stretched health system from becoming overwhelmed, infections must be brought down; if prevalence continues at the high rate we are seeing then hospitals will continue to be put under immense pressure, and more and more lives will be lost,” he added.
"We all have a part to play in preventing this situation from worsening and must do our best to stay at home wherever possible."
Health secretary Matt Hancock said the findings show "why we must not let down our guard over the weeks to come".
"Infections across England are at very high levels and this will keep having a knock-on effect on the already significant pressures faced by our NHS and hospitals,” he added.
"It is absolutely paramount that everyone plays their part to bring down infections."