A further 7,490 new coronavirus cases have been recorded in the UK.
Official figures show there were also eight more deaths on Sunday.
This is the fifth day in a row that new infections have exceeded 7,000.
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It comes as Boris Johnson is set to announce his decision tomorrow on whether he will lift lockdown restrictions completely.
The Prime Minister is expected to confirm on Monday that the final stage of easing controls in England - slated for June 21 - is to be put on hold for up to four weeks amid a surge in cases of the Delta variant first identified in India.
The move has angered some senior Tories, who have said there is no justification for another "catastrophic" delay to so-called "freedom day" when social distancing finally comes to an end.
However, Professor Andrew Hayward, a member of the Nervtag group which advises ministers on new respiratory diseases, said it was clear the country was facing a "substantial" third wave of the disease.
He said the key issue was the extent to which that led to more people becoming seriously ill and requiring hospital treatment.
He told BBC's The Andrew Marr Show: "We still don't know how bad it could be".
He said it was "extremely worrying" that the Delta variant - which now accounts for 96% of new infections - was proving to be 60% more transmissible than the previously dominant Alpha strain first identified in Kent.
He said: "That is the thing that will drive the speed with which the next wave comes along.
"I think if we were to open up more that would really fan the flames and lead to this increasing even faster."
Foreign Secretary Dominic Raab said ministers and officials were monitoring the data in "real-time" to determine when it would be safe to open up.