Three scientists advising the government on coronavirus have warned the easing of lockdown measures from Monday may be happening too early and could lead to a rise in infections.
The experts, all members of the Scientific Advisory Group for Emergencies (SAGE), have said ministers are taking risks by following their current plans.
Britain is set for a weekend of high temperatures with police urging the public to follow current advice ahead of the loosening of restrictions in England, which will allow up to six people to meet outside if social distancing of two metres is observed.
Downing Street has warned the public that these meet-ups remain prohibited until after the weekend.
Scientists have expressed concern that the NHS test and trace system to help stop the spread of COVID-19 is yet to be proven to be working effectively, and the reproduction rate of the virus is still only just below one.
This "R" number is the measure of how many people will be infected for every one person who has the disease.
Professor John Edmunds told Sky News: "Small changes can put that reproduction number up above one. The more important thing is that we still have a lot of cases.
"The number of infections that we have is about 8,000 new infections every day in England alone, and that's just in the community.
"There's more infections happening in hospitals and care homes and other enclosed settings, which wouldn't be included."
Professor Edmunds and fellow SAGE member Sir Jeremy Farrar have also said the test and trace scheme, which went live on Thursday, needs to have been in place for longer.
Sir Jeremy said the system needed to be "fully working" before the restrictions were relaxed.
Professor Edmunds said: "Track and trace was only launched the day before yesterday, so we can't be sure that that's working effectively, and yet we're going ahead and making these changes.
"I think that is rather dangerous.
"Even if we don't get a second peak and we just keep the incidents at this level, that would still result in large numbers of infections over time and people will die."
Sir Jeremy said in a Twitter post: "COVID-19 spreading too fast to lift lockdown in England. Agree with John and clear science advice.
"TTI (test, trace and isolate) has to be in place, fully working, capable dealing any surge immediately, locally responsive, rapid results & infection rates have to be lower. And trusted."
Professor Edmunds and Sir Jeremy also expressed concern about the gradual reopening of shops, schools and larger gatherings to meet in private while the number of cases is "relatively high".
It comes as a tranche of papers released by SAGE members has revealed advice given to the government in April, which said it was "likely" the "R" number would go above one should non-essential shops be reopened.
Asked if he agreed with Professor Edmunds and Sir Jeremy that it was too soon to ease lockdown restrictions, Professor Peter Horby told BBC Radio 4's Today programme: "Actually, I do.
"You know, we have still got 8,000 cases a day.
"We have been very successful in bringing it down, decreasing the numbers because of the social distancing.
"But, you will have heard that the R level is between 0.7 and 0.9, so it's only a bit below one, so, we have got very little headroom, actually.
"And, it's really important that we use that headroom very wisely and we don't lose control again".
Liberal Democrat health spokesperson Munira Wilson said: "The decision by key members of SAGE to go public with their concerns shows that ministers are no longer following the science.
"The test, trace, isolate system that we need to keep people safe is not yet fully functional.
"The NHSX app is delayed for an unknown period. For seven days straight the government has been unable to provide even basic data about the number of people tested.
"On top of these failings, public health messaging has been badly undermined as people see it's one rule for the Tory elite and another for everyone else.
"Given this chaos, measures to lift lockdown appear premature."
Next week from Monday to Thursday, Dermot Murnaghan will be hosting After the Pandemic: Our New World - a series of special live programmes about what our world will be like once the pandemic is over.
We'll be joined by some of the biggest names from the worlds of culture, politics, economics, science and technology. And you can take part too.
If you'd like to be in our virtual audience - from your own home - and put questions to the experts, email firstname.lastname@example.org