A leading scientist has warned that opening schools too early could spread the coronavirus “up the age groups”, saying that, at present, there could be an infected pupil in every class.

Professor Andrew Hayward, a member of the government’s Scientific Advisory Group for Emergencies (SAGE), said there needed to be a “continuing decline“ in community cases before schools could reopen to more pupils.

But the director of the UCL Institute of Epidemiology and Health Care added that the government could be saying more about its plans for how schools could reopen.

Speaking in a personal capacity on BBC Radio 4’s Today programme, Professor Hayward was asked whether he thought the prime minister was right to be cautious about bringing forward the full reopening of schools from the planned date of 8 March.

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He said the prime minister was “absolutely right to be cautious” and that the pandemic situation was still “very serious”.

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“We know that when we begin to release, the rates will start to bounce back very quickly,” he said. “And we know that, yes, it’s fantastic that we vaccinated 10 million people but there’s still a lot of vulnerable people yet to be vaccinated, so it’s too early to release just yet.”

But Professor Hayward added: “I think it would be good to see plans laid down, particularly in relation to schools, about…what sort of criteria you would use to think about reopening and how you would open them.

“I think that’s the most important thing, but the timing is always difficult to predict because we have to be responsive to the data. But how you open them is something that we could be saying more about.”

He was asked what he would look for in the data that might lead him to conclude that 8 March was too early to reopen schools, to which he replied: “I think we would be wanting to see [community cases] continuing to decline… that’s probably the most important measure...

“At the moment we might have one in 40 or one in 50 of us infected, and you probably have similar in school-aged children, so that’s going to be one in every class or two - so that’s pretty soon going to start to spread.

“And we also know that when infections do spread in schools they can contribute to broader community transmission and that will spread up the age groups over time as we’re trying to vaccinate down the age groups.”

Speaking at yesterday’s Downing Street briefing, Boris Johnson said 8 March was three weeks after the most vulnerable groups should have received the vaccine, by which time immunity should have set in.

He said: “What we don’t want to do, now that we are making progress with the vaccine rollout and we have got a timetable for the way ahead, we don’t want to be forced into reverse.”