The National Association of Schoolmasters/Union of Women Teachers said it would take immediate action if zones tried to introduce evening and weekend working or attempted to reform the school year.
Nigel de Gruchy, the union's general secretary, said: "We will watch them like hawks. Our members just have to come to us and we will give them full support."
The union's intervention threatens to halt some of the most radical proposals expected to be pioneered in the first 12 action zones, to be announced tomorrow.
Schools and local authorities which are bidding to form the first zones hope to introduce daily evening classes for pupils and their parents and Saturday schools.
Some bidders, such as the London Borough of Newham, want to replace the traditional three-term year with five terms and cut the six-week summer holiday enjoyed by generations of children and their teachers. Others propose a four-term year.
David Blunkett, the Secretary of State for Education, has made it clear the zones will be a test bed for future reforms, and has urged them to develop "innovative" proposals. A further 13 zones will open in January and more are planned in the future.
But Mr de Gruchy, who represents 172,000 serving teachers, said changes to working practices would provoke immediate demands for industrial action. "They will be outraged and there will be demands for action," he said.
"The situation is tinder dry and it just needs a spark to set the whole thing off. The Government has no idea how cheesed off teachers are."
Each zone will consist of about 20 schools under a special board of governors charged with thinking the unthinkable to raise standards. Each pounds 1m-a-year project will be able to tear up the national curriculum and teachers' pay and conditions agreements to raise standards and test radical reforms.
Plans for the zones were greeted with scorn by delegates at all three teacher union conferences this Easter, although the NASUWT leadership has been by far the most vociferous in its condemnation.
Graham Lane, education chairman of the Local Government Association, said teachers had nothing to fear from change, arguing reform could improve the lot of staff. He said: "If you are going to ask people to come in on Saturday mornings you have to ask for volunteers. We have had no problem recruiting Saturday teachers."
But Mr de Gruchy, who led this year's successful industrial action to force a reduction in teachers' workloads, said teachers could not be bought off with promises of extra pay.
He said: "It would require billions and billions of pounds to get them remotely interested and I don't think the country can afford it. The Government is playing with fire."Reuse content