Education Secretary Nadhim Zahawi told them to apply on the Get Into Teaching website from today, and preferably before Friday, for a January return.

       His plea comes after reports of low attendance at some schools of both teachers and pupils before the Christmas break.

       The Omicron variant is expected to cause even more absences in the spring term and unions have warned that lessons in classrooms may be affected.

       The Department for Education said more than 236,000 pupils and 2.1 percent of teachers were absent because of Covid last week. Some lessons have already moved online, according to reports from 35 local authorities.

       School and academy chiefs are preparing for a possible switching to online learning next term. Some pupils were asked to take laptops home before Christmas.

       The Government is pleading with recently retired teachers or those who changed career to "find even a day a week for the spring term".

       Mr Zahawi said: "I am asking any teachers no longer in the profession to come forward if they are available to temporarily fill absences in the new year.

       "Although 99.9 percent 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."

       Those eligible to apply can expect to receive targeted appeals from today, encouraging them to participate.

       Meanwhile, A-level students due to sit their final exams next year could leave without having taken a paper. Mock A levels and GCSEs, planned for next month have already been put in jeopardy by soaring Covid infections.

       Schools have been asked to hold three sets of mock exams in class as evidence of pupil performance, so as to award grades if national exams are scrapped for a third year next summer. If they are cancelled, it means the class of 2022 - having missed their GCSEs in 2020 and their A levels this year - would leave without taking a public exam.

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       In a government U-turn, GCSEs and A levels were scrapped soon after the start of last year's spring term as the country went into Covid lockdown.

       School heads are now calling for quick decisions and clear guidance, saying the uncertainty is disruptive for teachers and pupils.

       Headteachers' union the NAHT said the Government's plea to ex-teachers "sends a clear signal that we could be talking about a very different type of education at the start of next year, which has huge implications for things like exams and assessment".

       A spokesman for the Department for Education said: "As a last resort, there are contingency plans if exams can't happen, but we're nowhere near that being the case at the moment."