Leaders of the National Union of Teachers said that the vote for a ballot on a one-day strike would lead to a rolling programme of industrial action.
The narrow majority for a strike ballot at the NUT's annual conference in Blackpool follows a vote by the moderate Association of Teachers and Lecturers last week to ballot members on strikes over class sizes.
Delegates defeated a moderate amendment from the executive which supported local action when requested by individual schools with areas of oversized classes. Doug McAvoy, the NUT's general secretary, said: "This is not just a vote for a one-day strike. They see it as being the first of many one-day strikes to build up destructive action in schools."
The decision has yet to be finally confirmed by conference.
Earlier, militant teachers clashed with the executive when John Bills, the union's president, apologised unreservedly for the behaviour of delegates who mobbed David Blunkett, Labour's education spokesman, on Saturday.
There were cries of "rubbish" as Mr Bills accused the protesters, who besieged Mr Blunkett in a small office, of being "as bad as fascists". When he suggested that governors at the protester's schools might consider taking action against them for bringing their schools into disrepute, there were cries of "shame" and "disgraceful".
He said the incident had not only frightened Mr Blunkett, but must also have been frightening to the public and children in the vicinity.
Several of the union's branch secretaries have already received complaints against named members who took part in the demonstration. Mr McAvoy said that complaints would be investigated and disciplinary action could be taken if anyone was found to have brought the union into disrepute. The demonstrators could face expulsion.
Two union members involved in the protest who appeared on TV on Saturday night are Fran Crowhurst, from Lewisham, and Keith McKenna, from Birmingham. Both yesterday refused to speak to journalists.
Another protester, George Arthur, a primary school teacher from Barnsley, said he had been appalled by Mr Blunkett's statement last week that failing schools should be closed.
Many of the protesters wore Socialist Workers Party badges. Stefan Simms, an SWP member and Brent teacher who said he had watched the protest from a distance, said he thought it was appropriate action and accused the executive of starting a witch-hunt. He said: "No one pursued a blind man. People demonstrated. No one touched him."
During the debate on class size, delegates argued that the union's policy of backing members who refuse to teach oversize classes was not working. The union says ordinary classes should be no bigger than 30 and reception classes, 27. More than a million primary school children are in classes of more than 30. Teachers, parents and governors believe class sizes will increase because of this year's Government spending cuts.
Tim Foster, from Lewisham, said: "The Government must fund a reduction in class sizes or expect teachers to take strike action." Richard Rieser, from Hackney, said: "The number of children in classes is going up. The amount of money is going down."
Fran Postlethwaite, from Barnsley, told of a small rural school in Shuttleworth, Derbyshire, where all three teachers, including the head, were teaching classes of 40.
But John Cox of the executive warned the delegates not to disrupt the alliance with parents and governors who had supported teachers in their fight against spending cuts.
He said: "You lose parents' support when you call a national strike in a school where those parents have children who are not in oversize classes and where there is no underfunding."
The union's final position on action over class sizes will not be confirmed until tomorrow when it votes on a motion which will include the amendment calling for a national one-day strike.
Gillian Shephard, the Secretary of State for Education, last night warned teachers not to strike. "The education reforms are helping our children to achieve higher standards. It would be deplorable to put this progress in jeopardy and ridiculous to damage teachers' professionalism."
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