Ministers have been advised that changes to lockdown, including the reopening of schools, require a “highly-effective” track and trace system to be in place.

Professor Dame Angela McLean, the government’s deputy chief scientific adviser, told today’s Downing Street briefing that scientists had been “very clear” that changes to social distancing measures needed to be based on “observed levels of incidence in places that there’s going to be change, not on a fixed date”.

Asked whether it was “too early to conclude the science says that it’s safe to return to schools”, McLean said: “Scientists have been clear in our advice that changes to lockdown as we modelled them need a highly effective track, trace and isolate system to be in place.”

Ministers want pupils in reception, year 1 and year 6 to return to schools from June 1, but teaching unions have warned the date is too early for schools to reopen to more pupils safely.

The government has been developing a “test, track and trace” strategy to help stop the spread of the coronavirus, by identifying people who need to self-isolate.

However reports this week stated the programme has been delayed.

Therese Coffey, the work and pensions secretary, told BBC Breakfast this morning that the government’s contact tracing app, currently on trial on the Isle of Wight, would be ready for rollout in the “coming weeks”, but that she was “not aware that’s been set as a condition that’s necessary for the phased reopening of primary schools”.

But when asked about the new system this evening, McLean said scientific advisers had been “very clear that any change to the social distancing measures should be based upon observed levels of incidence in places that there’s going to be change, not on a fixed date”.

Asked about rumours the track and trace system would not be fully in place by June 1, Mclean said she would wait until she is updated later this week.

“So we’re getting a full update on Thursday of exactly what’s going to be in place and when, so perhaps you seem party to knowledge that I’m not party to. I’m going to wait to see what I get told on Thursday about what’s going to be in place by when.”