Reopening secondary schools could see the coronavirus reproduction rate increase “by as much as a half”, professor Neil Ferguson has warned.

Ferguson – the Imperial College epidemiologist who devised the model of infection that persuaded the government to introduce a national lockdown – said to mitigate this there would likely need to be “some tightening up” of restrictions “in other areas”.

Schools minister Nick Gibb confirmed on Wednesday the government’s position that all children will return to school in September – including in areas of local lockdown. But he added that the government cannot “decree” that keeping schools open must be prioritised over pubs during local lockdowns to combat spikes in coronavirus cases.

Speaking to BBC Radio 4’s Today programme, Ferguson said: “I think it is important to say that not all schools are the same. We have a lot of evidence now that primary schools, young children, pose very little risk of transmission.

“I think the concern is with secondary schools, teenagers, further education colleges and universities where the evidence is still not certain, but it looks like older teenagers can transmit just as well as adults.

“The risk then is that big schools, comprehensives, universities, FE colleges, link lots of households together, reconnect the social network which social distancing measures have deliberately disconnected. And that poses a real risk of amplification of transmission, of case numbers going up quite sharply.”

He added: “In terms of the reproduction value, the ‘R’ value, opening high schools could increase it by as much as a half, but by as little as 0.2 or 0.3, but it will go up.

“Given we’re at ‘R’ equal to one at the moment, clearly we don’t want ‘R’ going up to 1.5 or so, that would … lead to quite rapid growth of the epidemic.”

It came as the children’s commissioner for England, Anne Longfield, said the reopening of schools “should be prioritised”, insisting they must be first to reopen and last to close during any local lockdowns.