Unions predict majority of teachers ballotted will vote for a boycott of national tests for 11-year-olds

Headteachers in England were today gearing up for a battle with the next government – whatever its political colour – after it emerged that thousands plan to boycott national tests for 11-year-olds days after the general election.

Teachers' trade unions are balloting tens of thousands of school heads and their deputies over whether to boycott maths and English tests, formerly known as Sats, which 600,000 children are expected to sit during the week starting 10 May.

Teachers have until 16 April to vote, but today the unions – the National Union of Teachers (NUT) and the National Association of Head Teachers (NAHT) – said they were confident the majority of those balloted would vote for a boycott.

The two unions have asked teachers whether they would agree to "frustrate the administration" of the tests. Pupils and teachers would still attend school if a boycott went ahead, but children would not be entered for the tests.

A boycott could be the first battle a new government faces after the election, which is predicted to be called for 6 May.

Labour and the Conservatives support the tests. Last year Gordon Brown made a personal appeal to teachers not to boycott them. Ed Balls, the schools secretary, has announced reforms to next year's tests which will see teacher assessments published alongside the externally marked exams. The Tories are considering deferring the tests to the first year of secondary school and allowing teachers, rather than external examiners, to mark them.

But the unions want them scrapped, saying the tests are used to compile "meaningless" school league tables, which unfairly stigmatise schools with the most challenging pupils, and turn children's last year of primary school into a repetitive drill for the exams.

Christine Blower, the general secretary of the NUT, said: "If we have a successful ballot outcome, which we are confident that we will, then we will issue a list of instructions to teachers of things they should not do so, that way it will frustrate the administration of the tests."

In January, a quarter of NUT members responded to an initial ballot. Three-quarters of respondents voted for a boycott.

Teachers will be asked to pass a motion at the union's annual conference in Liverpool next month calling to abolish Sats.

The motion, proposed by teachers in Waltham Forest, east London, states: "We believe that we must move to a system where assessment is truly for learning ... League tables are the driving force behind the decline in nurturing a love of reading and a love of learning in children. Abolishing Sats is the first step in a move to genuine improvements in achievement."

The NUT is also heading for collision course with the next government over the expansion of academies. At the conference, teachers will be asked to step up plans to thwart Labour and Tory proposals to boost the number of academies.

A motion, proposed by teachers in Lambeth, south London, states: "Further expansion of the academies programme may tip the balance from a coherently planned, local and democratic education system into a wasteful patchwork of privatised, competing school market places in which those with 'buying power' will do best."