Angry teachers clashed with the Government last night as Britain's biggest classroom union threatened a national strike in protest at Tony Blair's school reforms.

The National Union of Teachers savaged the Prime Minister's controversial city academies programme and vowed to fight his Education Bill.

But the Government hit back, condemning the NUT's "retrograde" decision and accusing the union of putting children's education at risk.

Delegates at the NUT's annual conference in Torquay voted to step up their campaign against the Bill, which sets out moves to give business and faith groups a bigger role running schools.

They voted overwhelmingly in favour of a motion which raised the prospect of industrial action.

Martin Reed, from the NUT's national executive, said Education Secretary Ruth Kelly had misjudged the opposition to the reforms.

"Many in Government thought we would not fight," he said. "Big mistake Mrs Kelly, we will."

The teachers accused the Government of undermining comprehensive education, and demanded a national publicity campaign to persuade parents to oppose the reforms.

They passed a motion which called on the union's leader-ship to "explore the circumstances under which national industrial action, up to and including national strike action, could be taken to highlight the strength of opposition to the Education Bill".

The NUT should campaign "for support for such action among members", the motion said.

The union also threatened strike action on a school-by-school level to oppose the city academies programme.

Des Smith, a former headmaster, quit his job helping to recruit private backers for academies earlier this year after he allegedly suggested that they would receive honours in return.

Mr Smith was arrested and questioned by detectives investigating the "cash for honours" affair before being released on police bail.

The union warned that the extra freedoms given to academies could damage teachers' pay and conditions.

And delegates turned their fury on the "cash for honours" row.

Jane Bassett, from Hackney, east London, urged teachers to fight "a Govern-ment that's prepared to sell peerages to academy sponsors".

Liam Conway, from the NUT's central Nottingham-shire branch, said: "The Bill is the academies programme writ large. The union should be considering pre-emptive industrial action to stop it."

Hazel Danson, from Kirklees, won a standing ovation from the 1,000-strong conference when she attacked Government interfering in the way teachers teach.

"The guardian of standards in my classroom is not Ruth Kelly," she said. "The guardian of standards in my classroom is me."

Speaking after the vote, NUT general secretary Steve Sinnott promised to look at the possibility of taking industrial action in opposition to the Bill.

A spokesman for the Department for Education and Skills condemned the NUT's

vote. "Strikes achieve only one thing - disruption to pupils' learning," he said. "This is a retrograde step based on arguments that don't stack up.

"The NUT have opposed many significant education reforms which have subsequently brought about the highest ever standards in schools.

"There is nothing to fear and everything to gain from the Education Bill."

The Government's reforms propose a new generation of "trust schools", backed by business and faith groups, with more freedom from local authority control to set their own admissions policies.

The business or faith group backers could have a major say in the way a trust school is run, including appointing governors and giving the school a distinctive ethos, under the plans.

City academies are separate from the Education Bill proposals. Ministers want to create 200 of the privately sponsored schools by 2010 and 27 are open already.