Education Secretary Nadhim Zahawi is asking them to temporarily help plug gaps in order to maintain face-to-face teaching.

The rapid spread of the Omicron variant is expected to force an increasing number of teachers to isolate and also deplete the number of supply staff.

Some schools are already preparing for the possibility of online teaching and have told pupils to take laptops home in case of disruption after Christmas.

It is hoped that teachers who have retired or moved profession will sign up from today, with the government saying even just one day a week would be helpful.

People are being urged to get the process started now so that the necessary checks are completed by January.

"Although 99.9% of schools have consistently been open this term, with cases of Omicron increasing we must make sure schools and colleges have the teachers available to remain open for face-to-face education," said Mr Zahawi.

"Anyone who thinks they can help should get the process started now on the Get Into Teaching website, and everyone should get boosted now to help reduce the amount of disruption from the virus in the new year."

Qualified primary, secondary and further education teachers are all being asked to heed the call.

Supply agencies will also continue to manage local areas to ensure schools don't need to close due to lack of staff, said the Department for Education.

Sir Michael Wilshaw, the former head of Ofsted, backed former teachers helping out, saying he went into a north London school during the first COVID wave and he will be helping at a school in south London in January.

The former headteacher, who is over 70, told Sky News he had to weigh up the health risks of travelling on the Tube but said he was healthy for his age.

"You don't lose the skills of teaching, that ability to engage with children. It's a bit like riding a bike," he said.

Sir Michael said it is "an individual decision" and members of his family advised him not to go into schools "but I'm glad I did".

"I feel it's a moral imperative," he added.

Labour's shadow schools minister Stephen Morgan said the move was "a sticking plaster, and only part of what's needed to keep children and staff safely in class next term".

"The government's failure to get a proper workforce plan in place leaves staff, children and parents relying on goodwill from retired staff and volunteers, many of whom face additional risks themselves," he added.

Follow the Daily podcast on Apple Podcasts, Google Podcasts, Spotify, Spreaker

Paul Whiteman, head of the NAHT union, said having more teachers available "could be helpful" but said schools were already dealing with "very challenging circumstances".

"We need to be very clear that if things get to this stage, it will mean that education will look very different in January and we could be talking about a very different type of provision at the start of next year," he said.

"That has huge implications for things like exams, assessment and inspection."