A scientific adviser to the Government has warned that the UK could still suffer more than 100,000 deaths by the end of the year if measures are hastily relaxed.
The unnamed advisor told the Sunday Times: "There is very limited room for manoeuvre."
It is understood that warnings about the potential death toll were sent to the government's Sage advisory committee early last week by researchers from the London School of Tropical Hygiene, Imperial College and other centres.
The experts modelled different lockdown exit policies "to evaluate which were viable and which were not," a scientific adviser said.
The source told the paper more than one model had put the death toll in six figures in some scenarios.
Boris Johnson acknowledged the scale of the danger, saying "we'll have to work even harder to get every step right" now the peak is passed, before making a mountaineering analogy.
"You have very few options on the climb up - but it's on the descent you have to make sure you don't run too fast, lose control and stumble," he told the Sun on Sunday.
It is understood that a warning system administered by a new "joint biosecurity centre" will detect local increases in infection rates, with a view to locally altering restrictions in England.
With the alerts ranging from green in level one to red in level five, Mr Johnson is expected to say the nation is close to moving down from four to three.
On Monday, the Government will publish a 50-page document outlining to MPs the full plan to cautiously re-start the economy after figures suggested the overall death toll for the UK has passed 36,500.
The shift in messaging will come amid concerns that workers may not feel comfortable resuming their roles after the weeks of firm instructions to "stay at home, protect the NHS, save lives".
Professor Peter Horby, chairman of the UK Government's New and Emerging Respiratory Virus Threats Advisory Group (Nervtag), said ministers should be "incredibly cautious" about any easing of the lockdown measures.
Speaking to the BBC's Andrew Marr Show, he said: "We have to be clear that this is not like a storm where we batten down the hatches and then it passes by and we walk out into the sunshine and it's gone.
"It's still out there. Most of us have not had this virus. So if we get this wrong it will very quickly increase across the population and we will be back in a situation of crisis.
"So we have to be incredibly cautious about relaxing the measures."
While Tarik Jasarevic from the World Health Organisation, urged governments to be careful when removing restrictions saying that it would be "traumatising" for people if they had to return to lockdown conditions should there be a second wave of infection.
He told Sky's Sophy Ridge On Sunday: "This is the advice we give really to every country is to be very, very cautious when taking these decisions to ease down the restrictions, because really what we don't want to see is that there is a second wave and that we have to go back to the lockdown. That would be traumatising I think for everyone.
"So it's important that this is done in a staged, in a gradual way, so we can really see what is the effect of people getting back close to one another? So it is really important that this is being done very, very carefully, so we avoid the possibility that we have to go back into a situation we've been in."
