A leading scientist today warned it was still “uncertain and debatable” whether it is safe to reopen schools after the Government said it hoped that primary-age children could start to return next month in class sizes of no more than 15.

Epidemiologist Professor Paul Hunter, a specialist in infectious diseases at the University of East Anglia, said the lack of clear evidence about the safety of pupils returning to the classroom meant it should only be done “very carefully”.

He told BBC Radio 4’s Today programme that ministers need to look closely at the impact of school reopening programmes in continental Europe. He said so far there had been “mixed messages” on whether they contributed to a resurgence in the epidemic.

Boris Johnson said in his televised address on Sunday that children in Reception, Year 1 and Year 6 in primary schools might begin to return from 1 June “at the earliest”. Further guidance yesterday revealed plans to widen this to all primary year pupils, but with a warnings of delays if “insufficient progress is made in tackling the virus”.

Details set out by the Department for Education show classes will be divided into groups of no more than 15 pupils — and these small groups will not mix with other pupils during the school day

The guidance says children should be kept two metres apart if possible.

There will be staggered break and lunchtimes, and different times for starting and finishing the school day.

But the National Education Union has rejected the reopening plans as ­“reckless”.

Parents who choose to keep their children at home will not face fines.