The UK is "not out of the woods yet" and people should act with caution as Covid restrictions in England end on Monday, Prof Chris Whitty has said.
England's chief medical officer warned that Covid hospitalisations were doubling every three weeks and could hit "scary numbers" in future.
Prof Whitty said the pandemic still had a "long way to run in the UK".
Solicitor General Lucy Frazer said while cases will rise, there were "consequences for not opening up".
But she warned: "Of course, if we get into a situation where it is unacceptable and we do need to put back further restrictions, then that of course is something the government will look at."
It comes as:
- the Office for National Statistics (ONS) estimated 577,700 people in the community had Covid in England in the week ending 10 July - equal to one in 95 people
- the UK recorded almost 50,000 new cases on Thursday - the highest daily number since January
- more than half a million self-isolation alerts were sent to people using the NHS Covid-19 app in England and Wales during the first week of July - a rise of 46% on the previous week
- Meat processors warned of product shortages due to staff absences, driven by "pings" sent from the app
The ONS figures for England represent a rise on the previous week, when one in 160 people were estimated to have the virus.
Prof Whitty told an online seminar hosted by the Science Museum on Thursday evening that the situation in the UK could become very dangerous for individual hospitals.
He said: "I don't think we should underestimate the fact that we could get into trouble again surprisingly fast.
"I think saying the numbers in hospital are low now, that does not mean the numbers will be low in hospital in five, six, seven, eight weeks' time.
"They could actually be really quite serious."
He added: "We've still got 2,000 people in hospital and that number is increasing. If we double from 2,000 to 4,000 from 4,000 to 8,000, 8,000 and so on it doesn't take many doubling times until you're in very, very large numbers indeed."
While Prof Whitty cautioned "we are not by any means out of the woods yet", he added that "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," he said.
But Prof Whitty said it was crucial that from 19 July in England, people "take things incredibly slowly" - adding that he anticipated most people would still take precautions.
"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, I'm going to reduce my contacts, I'm going to improve ventilation...'," he said.
From 19 July, almost all legal restrictions on social contact will be removed in England, but some guidance will remain. Different rules apply in the the rest of the UK.
Nightclubs will be allowed to reopen for the first time since March last year and capacity limits will be removed for all venues and events.
There will no longer be any limits on how many people can meet and the 1m-plus distancing rule will also end.
The legal requirement to wear face coverings in some enclosed public places will expire, but Health Secretary Sajid Javid has previously said masks would still be "expected and recommended" in crowded indoor areas.
But the UK's two largest supermarkets - Tesco and Sainsbury's - have said they will continue to ask shoppers to wear masks from Monday to protect staff and shoppers.
Prof Whitty predicted that in the medium term, coronavirus could mutate into a "vaccine escape variant" that could take the UK "some of the way backwards" into the worst days of the pandemic.
"The further out in time we go, the more tools we have at our disposal from science, the less likely that is but you can never take that possibility completely off the table," he said.
"But you know, science has done a phenomenal job so far and it will continue to do so."
Prof Whitty's comments echo his statements at Monday's Downing Street coronavirus briefing where Prime Minister Boris Johnson confirmed the final stage of unlocking in England would take place on 19 July.
The chief medical officer said the "overwhelming view" of the scientific community was that moving "slowly" through the next step of easing restrictions was "essential".
He added that while the numbers of people being admitted to hospital with Covid were "not trivial", they were rising at a much lower rate than previous waves.
Explaining the government's approach, Ms Frazer told Sky News: "It is really important that we get the balance right between ensuring that we keep this virus under control and we take the necessary clinical measures to do that, but that we also recognise that there are consequences of not opening up and not allowing people to go about their daily lives."
She added that "a large number of people have been vaccinated, we've had a really tough time, we're still asking people to take responsibility and we do need to ask ourselves, if we don't open up now, when will we be able to open up?"
On Thursday, the UK recorded new 48,553 new infections and a further 63 deaths within 28 days of a positive test.
Delta virus variant cases rose by 17% in the latest week, with a further 36,800 recorded, Public Health England (PHE) said.
The variant continues to account for approximately 99% of confirmed cases of coronavirus across the UK.
The ONS estimated one in 90 people in Scotland had the virus in the week to 10 July, one in 360 in Wales, and one in 290 in Northern Ireland.
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