Boris Johnson should resist calls to ease the lockdown including reopening schools until a new system to trace the spread of coronavirus is a proven success, according to two members of the government's scientific advisory council.
The prime minister must decide in the next 10 days whether to reopen schools and allow those in non-essential jobs to return to work at the start of June, something he set as an ambition in an address to the nation 10 days ago.
However Sky News understands that the government has been warned by its scientific advisers not to proceed until the "contact tracing" system is up and running and, importantly, is a proven success.
Downing Street has not said when contract tracing will start, and it has not given many details of how it will operate.
One Whitehall official said that "lots of work" is going on to have it ready as soon as possible, and they hoped to have a start date "in a few days".
A smartphone app, which will eventually form part of the contact tracing process, is unlikely to be ready for several weeks.
Justice Secretary Robert Buckland told Sky News the government hopes track and trace systems will be up and running by the "end of the month" and that he would be "frustrated and disappointed" if they were not operational by mid-June at the latest.
Asked about the date for reopening schools he said: "We always said 1 June was conditional, not just on the R rate, but also on the need to make places of work safe.
"We've got to accept the fact that some councils are employers and decisions have to be made collectively.
"There will be other institutions who believe it is safe to open up, but I utterly respect those who want to exercise greater caution."
The British Medical Association (BMA) on Wednesday changed its advice on schools, saying they could reopen on 1 June "if safe to do so", but added that it would involve an "accepted level of risk", as avoiding all threat of the virus would not be possible.
The doctors' union had previously said the government should not consider reopening schools in England until the case numbers are "much lower".
Professor John Edmunds is dean of the faculty of epidemiology and population health at the London School of Hygiene and Tropical Medicine, and a world authority on mathematical modelling of the spread of infectious diseases.
He said that a "well functioning track and trace system" was needed before lifting lockdown further, telling Sky News: "Without that, we would return to a situation where the epidemic is increasing."
He said it was "a very hard thing to achieve a well functioning" system, requiring a "huge amount of manpower" and technology.
Professor Edmunds added that evaluating the system was just as important - something there is little time to do.
"Then you need to know whether this is actually working or not. How many cases are being identified who are already in quarantine - what fraction of cases popping up already in quarantine - that would be a good measure of how well contact tracing is occurring," he added.
Pushed on whether the system was needed before schools should start to reopen, he said that "I think it would be greatly preferable".
The SAGE member, who said he was speaking in a personal capacity, added: "The general approach is we need to relax these restrictions very gradually. Very incrementally. And then evaluate how well we're doing every step of the way, each time we make a step we need to evaluate how we're doing before we move to the next step.
"There are certain things I think we need - it would be greatly preferable to have them in place before we start relaxing - the track and trace being one of those."
Another member of SAGE, who did not want to be identified, said that before lockdown is significantly eased, they thought that the government would do three things, firstly improve further the quality of the data around the R rate, secondly to reinforce the existing social distancing advice and thirdly a "capable" contact tracing system.
They said the contact tracing system was "a bit of a problem", and this was "pretty close to what SAGE is saying".
The challenge is to ensure a speedy system to find people diagnosed with coronavirus, identify who they have come into contact with, and get them tested - ideally within 24 hours.
"It's about speed of tracing and speed of testing that's really important," they said. The system must be able to trace more than half, perhaps as many as three quarters of contacts to be a success.
Other countries like Singapore have access to real time credit card data and facial recognition CCTV to help tracers identify contacts of those infected.
Such options are not available in western countries, which is why France and Germany are also struggling with track and trace schemes, the SAGE member said.