thousands more teachers at schools across the country could soon vote to withdraw cover for absent colleagues and vacancies, raising the prospect that the entire schools system could be plunged into chaos.
Further ballots are set to take place following the vote by members of the National Union of Teachers and National Association of Schoolmasters Union of Women Teachers in Greater London and Doncaster. Both voted overwhelmingly this week to withdraw cover for unfilled posts or staff absences of more than three days.
The unions predicted that about 1,000 schools in London and Doncaster will be forced to send pupils home for part of the week.
While ministers condemned the move as no solution to teacher recruitment problems, union leaders claimed that it was the only way to protect their members from exploitation and show parents the extent of the shortage.
But it could herald a long and bitter war of attrition, said David Hart, general secretary of the National Association of Head Teachers.
He said: "I understand the reasons for the action but it is not going to solve he problems of supply in the short-term. We are already scraping the barrel of supply teachers and raiding the ex-colonies.
"To avoid sending children home and facing the antagonism of parents, schools will be driven to even more desperate measures, using anyone who will stand up in front of a class."
Nigel de Gruchy, NASUWT general secretary, said teachers would co-operate to keep disruption to a minimum. He suggested teachers would cover lessons during the week but not on Friday afternoons.
The employers have threatened to dock the pay of staff who break their conracts. Teachers are required to cover three-day absences and longer in "exceptional circumstances". Union leaders argue that exceptional is now often the norm. They are due to meet the employers in the next few days to discuss the dispute.
More than 30,000 teachers were balloted and about a third voted. More than 90 per cent were in favour.
Further ballots are due in Portsmouth, Southampton, Leicester, Middlesbrough, Nottingham, Kent, Manchester and Reading and at a number of individual schools.