Boris Johnson’s plan to fully lift Covid restrictions in England has been criticised by scientists who called into question the effectiveness of making measures such as mask-wearing voluntary.
Other updated guidance warns vulnerable people to consider whether they need to meet those who are unvaccinated.
In a change of tone to previous promises that he would press ahead with “irreversible” lockdown easing, Mr Johnson urged caution as he said on Monday his priority was to protect the public and the National Health Service.
“I cannot say this powerfully or emphatically enough, this pandemic is not over,” he said.
“This disease continues to carry risks for you and your family. We cannot simply revert instantly on Monday, 19 July, as it was before Covid.”
Chief medical officer for England Chris Whitty said the country was bound to experience an “exit wave” of infections as the country is freed from restrictions.
“The slower we take it, the fewer people will have Covid, the smaller the peak will be, and the smaller the number of people who go into hospital and die,” he said.
But Prof Graham Medley from the London School of Hygiene and Tropical Medicine said on Tuesday that thousands could be admitted to hospital per week during the exit wave.
“We’ve never seen a peak before that hasn’t been controlled. In the previous peaks, we’ve had them go up and come down very sharply because we’ve introduced a lockdown,” he told the BBC Radio 4 Today programme.
“If the intention is to not introduce a lockdown for this peak, we are going to see a natural peak and that may well be long and disseminated. Even if we don’t get up to very high numbers, the numbers we have might last for several weeks, six weeks or so, in which case there’s still a considerable burden for health care.
“We may get more than 2,000 admissions per day. If that lasts over six weeks, then that’s a lot of people.”
Prof Medley, who is the chief modeller for the government’s committee of scientific advisers, said measures such as mask-wearing were only effective if the overwhelming majority of the public complied.
“Without the mandation, I think we will be in a situation where even if the majority of people, let's say 70 per cent of people wear a mask, will that do any good because of the 30 per cent who don’t?” he said.
Manchester Mayor Andy Burnham criticised the government for “mixed messages” on mask-wearing and the advice telling the vulnerable to avoid the unvaccinated, which he said would not work. He questions how vulnerable people were supposed to be able to identify if another person had been vaccinated.
"I think there was a sensible shift in tone yesterday,” he told Sky News.
"All that said to me was they've recognised the problem they've created with this rhetoric. Was it enough to fix the problem? I think not."
He said vulnerable people would not feel safe when legal enforcement of social distancing is removed.
"I expect to get complaints from people who feel unable to go to vital hospital appointments," he said.
"One person's freedom day is another person's fear day."
The UK’s move to loosen Covid restrictions comes as several European countries, including France and Italy, tighten curbs on people to stop the spread of the Delta variant.
Dr David Nabarro, the World Health Organisation’s special envoy on Covid-19, said the "pandemic is advancing ferociously around the world" and "I don't think we've anywhere near got through the worst of it".
"All this doesn't quite fit with the position that was taken by Britain, along with other nations, some months ago when there was a real effort to try to prevent large numbers of people getting the disease, partly because of the risk of death and partly because of the recognition of the risk of long Covid," he said.
"It's necessary to be unequivocal on this particular challenge. What does 'urging caution' mean? It's important that everybody knows the best possible advice on how to prevent themselves being infected."
He said it was not the right time to ease restrictions in the UK, where cases exceed 30,000 a day.
"I accept that vaccination has changed the nature of the equation in the UK but, quite honestly from any point of view, it's too early to be talking about massive relaxation or freedom when the outbreak curve is on such a sharp ascent," he said.
"Yes, relax, but don't have these mixed messages about what's going on. This dangerous virus hasn't gone away, its variants are coming back and are threatening those who have already been vaccinated – we have to take it seriously."