Reopening schools next month is likely to "put pressure" on the R rate, the Government's chief scientific advisor has said.
Sir Patrick Vallance said schools starting to open from June was likely to push the rate of coronavirus infections up.
"The risk for children (from coronavirus) is much lower – we know that," he told the Downing Street briefing on Friday.
"They are at low risk but not zero risk and there have been some serious cases of children, of course, but very few compared to adults and older age groups.
"The broader risk in terms of opening schools is that as soon as you introduce any contact, you put pressure on the R and you put pressure on numbers, and that’s true of anything we are going to do in terms of changes to contact."
It comes as new evidence shows children could be half as likely to catch coronavirus as adults and teachers do not appear to be at greater risk than other professions.
The research emerged on Friday in papers assessing the impact of relaxing school closures from the Goverment’s Scientific Advisory Group for Emergencies (Sage).
A review of global studies led by University College London found those aged under 20 had 56 per cent less chance of being infected.
However, evidence on how likely children are to transmit Covid-19 remains “inconclusive”, according to the scientific advice used to inform the proposals to reopen schools.
The paper published by Sage models nine different options for getting pupils safely back into the classroom.
These ranged from keeping them shut to all but vulnerable children or those of key workers, right through to fully reopening all primary and secondary schools.
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But the document says: "Evidence remains inconclusive on both the susceptibility and infectivity of children, but the balance of evidence suggests that both may be lower than in adults."
Teaching unions and some councils have spoken out against the Government’s plans to begin a phased reopening of schools in England on June 1.
In response to the publication of the Sage documents, the National Education Union suggested Boris Johnson was taking a “cavalier attitude” towards children.
Dr Bousted said it remains the case that the National Education Union does not yet think it is safe for the wider opening of schools.
Dr Patrick Roach, general secretary of teachers’ union NASUWT, added that the scientific advice would “only add to teachers’ uncertainty and anxiety”.
“The committee states that large-scale community testing is needed to better understand and monitor the prevalence of and susceptibility to Covid-19 in children, yet the Government’s plans for the reopening of schools from June 1 are premature whilst a widespread community testing system will not be in place," he said.