Charles Clarke is facing a growing crisis in the government's relations with the powerful teaching unions.

The NUT has voted for strike action over a massive expansion of classroom assistants, class sizes and national curriculum tests.

Delegates at the union's annual conference also backed proposals to limit class sizes to 27 by sending pupils home on a rotational basis.

General secretary Doug McAvoy admitted the move, forced through by left-wing union members, would make it "very difficult" to persuade parents that the union was protecting their children.

But addressing delegates, McAvoy launched a blistering attack on Clarke, saying his department was restricting dialogue between the union and government officials.

"It crosses a line that threatens democracy," he warned, adding that the education secretary was setting back industrial relations by 120 years.

The latest row follows other concerns that the complex system for school funding has left head teachers facing a budget squeeze despite government claims that more money than ever is being directed into the education sector.

Having refused to attend the NUT conference, Clarke will seek to regain control of the agenda when he addresses the NASUWT's annual gathering in Bournemouth later this week.

The education secretary faces the challenge of persuading grassroots union members to step back from calls for industrial action, while arguing that the school reform package will help tackle long standing concerns such as workload and red tape.

The reform deal, agreed after months of negotiations, has already been backed by ministers, local authorities and other leading teaching unions.

But the NUT has refused to give its support because of the greater classroom role envisaged for assistants from September 2005, a move designed to allow teachers to spend more time marking and preparing lessons during school hours rather than at home in the evenings or on weekends.

While the NUT has been isolated in its opposition to the deal, the government has been warned that many of its other concerns are valid.

David Hart of the National Association of Head Teachers said the NUT was "touching a nerve of public concern about testing and league tables".

Under pressure to tackle the crisis in relations between teachers and ministers, Clarke faces the challenging task of needing to reconcile opposing views within the union movement.

While the NUT has threatened to strike if the reforms are introduced, Eamonn O'Kane of the NASUWT has warned that his union could take industrial action if there are attempts to "frustrate the implementation" of the deal.

The problem comes at a difficult time for the Labour Party, which has been seeking to improve strained relations with the union movement.

It faces a new generation of left-wing leaders taking control of key unions.

The government has been struggling to balance the needs of business leaders and workers across a range of areas such as employment rights, the contracting out of public services and use of the private finance initiative.