Schools may need to close to some year groups in order to get control over the coronavirus infection rate, a former government adviser has warned.

Prof Neil Ferguson, whose advice led to the lockdown in March, said the current restrictions on household mixing "should have a significant effect".

But he said beyond that there was a "limit to what we can do" without sending some school year groups home.

Prof Ferguson also said relaxing the rules at Christmas would cause deaths.

It comes as the UK government announced on Friday there had been a further 20,530 confirmed cases and 224 new deaths.

More than seven million people in England are now living under the top level of coronavirus restrictions, with South Yorkshire the latest region to have new rules come into force.

The whole of Wales - 3.1 million people - is also seeing its first full day of national lockdown, which is due to last for 17 days.

Speaking on BBC Radio 4's Today programme, Prof Ferguson - who quit his role from the government's Scientific Advisory Group for Emergencies in May after breaking lockdown rules - said the situation was "worrying".

"We now have 8,000 people in hospital with Covid. That is about a third of the level we were at at the peak of the pandemic in March," he said.

"If the rate of growth continues as it is, it means that in a month's time we'll be above that peak level in March and that is probably unsustainable.

"We are in a critical time right now. The health system will not be able to cope with this rate of growth for much longer."