A union has called on the Government to “draw back” on the planned reopening of schools in England from Monday, saying prominent experts speaking out about the risk of easing lockdown too soon “changes everything”.

The National Education Union (NEU) said the contact-tracing system must be running successfully before the risk level that comes with children and teachers returning to school can be mitigated.

In a statement issued on Saturday afternoon, just 36 hours before schools are due to reopen to more pupils, the union said the comments from expert advisers undermine the Government’s claim that it is following the science.

This weekend, Professor Peter Horby, chairman of the New and Emerging Respiratory Virus Threats Advisory Group (NERVTAG), joined scientists Sir Jeremy Farrar and Professor John Edmunds, all members of the Scientific Advisory Group for Emergencies (Sage), in warning that ministers are taking risks.

Even at this late stage, we call on the Government to draw back from wider opening of primary schools from Monday

Another Sage member, Professor Calum Semple, insisted that a “brave” political decision had been taken on schools reopening “were everything normal”, but cautioned that high levels of transmission were still being seen, the BBC reported.

He said: “Essentially we’re lifting the lid on a boiling pan and it’s just going to bubble over.

“We need to get it down to simmer before we take the lid off, and it’s too early.”

NEU general secretaries Kevin Courtney and Dr Mary Bousted said: “No-one can now confidently assert that it is safe to open schools more widely from Monday.

“Opening schools more widely runs the risk of increasing the R rate and therefore the level of risk to staff and to parents.

“That risk can only be mitigated if contact tracing is running successfully.

“We have made that case strongly to Government – and we have been supported by the BMA and by the independent Sage group in our concerns. Government replies that it is following the science. But this public break by senior members of Sage, including by the chair of the NERVTAG committee, undermines that claim.

“School leaders, their staff and pupils’ families deserve better than this.

“Even at this late stage, we call on the Government to draw back from wider opening of primary schools from Monday.

“Instead we urge Government to engage in talks with the profession and the unions, including the NEU, about how to open schools more widely once the contact-tracing system is shown to be working.”

Meanwhile, three quarters of schools intend to reopen to more pupils in June, a poll carried out earlier this week suggested.

Most school leaders (77.6%) said they will take a flexible approach using rotas and a smaller number of year groups when they increase their numbers from Monday and the week after, the NAHT school leaders’ union said.

But 10% of the 833 school leaders polled said they will not be in a position to take more pupils on Monday, or the week after.

Just 12.1% said they will increase the number of pupils attending school completely in line with the Government’s proposals, the union said.

Earlier this week, Boris Johnson announced that the Government’s five key tests required for the easing of the lockdown have been met – and schools will admit more pupils in Reception, Year 1 and Year 6 from Monday.

Secondary schools will also start offering face-to-face contact from June 15, the Prime Minister said.

The decision came after teaching unions and council leaders spoke out about safety concerns.

On Saturday, NAHT general secretary Paul Whiteman said school leaders had worked “tirelessly” in recent weeks to put in place safety arrangements for pupils, staff and parents.

He said: “We have pushed hard for flexibility from the Government around the June 1 date and for a willingness to take local circumstances into account.

“Without it many schools could not take the measures advised by Public Health England.

“It is encouraging to see that the majority of schools have used positively the flexibilities afforded to them.

“Much of the angst surrounding the widening of access could have been avoided if this approach had been supported by Government earlier.”

He said schools will inevitably have staff who cannot work because they are shielding or have health issues, and that others might “not have enough confidence to go back”.

He added: “The Government now needs to use all of its powers to inform parents that their cooperation with local flexibility and adherence to public health advice will be essential to keeping schools safe.”

Headteachers in some parts of the country said they expect supermarket-style queues outside the school gates and divided playground areas when some pupils return.

Schools, colleges and nurseries closed more than nine weeks ago due to the Covid-19 outbreak, remaining open only for vulnerable youngsters and the children of key workers.

Findings from a PA survey of local authorities show that more than a dozen councils are advising schools not to open to more pupils from Monday.