Government suggestions that money could be cut from budgets by having fewer school leaders have fired up heads to support a boycott of next year's Sats, a heads leader has claimed.
Headteachers who are angry at comments made by Ed Balls, Schools Secretary, about cost savings have said they will support a campaign to stop key stage 2 tests being taken in May.
The revelation comes as thousands of teachers began receiving papers for an indicative ballot on whether they will support a boycott.
Teaching unions the NUT and the National Association of Head Teachers (NAHT) are calling for members not to participate in the tests unless the Government delivers fundamental changes to the testing regime.
The unions want to abolish testing for all Year 6 pupils and replace it with a system of teacher assessment. This would allow schools to offer a wider and more balanced curriculum, they claim.
Earlier this year, the NAHT admitted that heads were concerned about going through with the boycott because it could be illegal.
But Mick Brookes, the union's general secretary, said this week: "I have been to the regions to meet members and there is a much greater appetite to take this on than I thought.
"It was sharpened after Ed Balls' extraordinary comments about sacking heads at the Labour party conference. People were so outraged by that that I think it will increase the likelihood of people taking action."
At the time of the Labour conference in September, Mr Balls said in interviews that money could be saved by schools federating. Part of the savings would be achieved by pooling leadership roles across a number of schools, which would result in fewer jobs.
More than 25,000 people have signed a petition calling for the end of Sats, it was announced this week, as industrial action over the tests moved a step closer.
The two unions passed resolutions at their annual conferences earlier this year proposing to take joint action to boycott the tests if they were not scrapped.
Results of an indicative ballot of NUT members and a consultation with NAHT members are expected in early December. A full ballot would be needed next year if the boycott is to go ahead.
"If both ourselves and the NAHT were determined, and we hadn't got sufficient change, it would be impossible for the tests to be administered," said Christine Blower, NUT general secretary.
Mr Brookes rejected suggestions by Mr Balls that teachers should not rehearse children for the tests.
"That is like telling an Olympic high-jumper not to practise before the Olympics," he said.
The timing of any boycott would prove embarrassing for the Government as it will coincide with the date considered most likely for the general election.
Vernon Coaker, schools minister, said: "A boycott of statutory tests would be disruptive to pupils and risk doing damage to the standing of the profession.
"It is out of step with what parents want, not backed by teaching unions, and would be a breach of heads' statutory duties if the tests do not go ahead."