Chris Whitty has warned that England could be put back into lockdown if hospitalisations continue to rise.
The Chief Medical Officer warned that the number of people in hospital with coronavirus could reach "quite scary" levels within weeks.
Professor Chris Whitty said the UK is "not out of the woods yet", just hours after Boris Johnson declared it was "highly probable" the worst of the pandemic is over.
READ MORE: Covid patients 'occupy third of critical care beds' in Liverpool as cases rise
The latest figures showed 48,553 more cases, the highest since January 15, while the 63 deaths were the biggest daily reported increase since March 26.
Prof Whitty, England's chief medical officer, said: "I don't think we should underestimate the fact that we could get into trouble again surprisingly fast."
He warned that the number of people in hospital with Covid-19 is currently doubling about every three weeks and could reach "quite scary numbers" if the trend continues.
He added that more people could be fighting the disease in hospital "in five, six, seven, eight weeks' time", and went on: "They could actually be really quite serious... at that point if it looks as if things are not topping out, we do have to look again and see where we think things are going. "
Speaking at a webinar hosted by the Science Museum, Prof Whitty said: "We are not by any means out of the woods yet on this, we are in much better shape due to the vaccine programme, and drugs and a variety of other things.
"But this has got a long way to run in the UK, and it's got even further to run globally."
His comments came after Mr Johnson urged people not to "throw caution to the winds" as England's restrictions ease on Monday and acknowledged there would be more hospital admissions and deaths from Covid-19 to come during "difficult days and weeks ahead".
Prof Whitty said the key on July 19 was "to take things incredibly slowly", adding that he fully expected most people to continue to take precautions.
He said: "If you look over what people have done, and in fact if you look at what people intend to do now, people have been incredibly good at saying, 'I may be a relatively low risk, but people around me are at high risk, and I'm going to modify my behaviours'."
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