Teachers will refuse to hand out Sats papers

Heads and teachers are planning a campaign of industrial action to stop national curriculum tests for 600,000 11-year-olds.

The National Union of Teachers and National Association of Head Teachers will refuse to deliver the tests in maths and English. Their action, scheduled to start on 10 May, is expected to lead to widespread cancellations. The two unions represent 85 per cent of heads in the 16,000 schools where children will sit the tests.

The action will ensure that the future of the tests is the first item in the in-tray of any new government. NUT members are also threatening strikes to stop an expansion of the academies programme being planned by the Conservatives. Michael Gove, the shadow Education Secretary, has said he will bring a "rocket charge" to the programme and launch up to 100 new academies within 100 days of the election of a Tory government.

In particular, he wants schools that are rated outstanding by Ofsted, the education standards watchdog, to be able to set up as free-standing academies by September. And for the first time primary schools will be allowed to opt for academy status – and be run by private sponsors.

The union is also planning a separate campaign to stop Tory plans to set up a network of Swedish-style independent "free" schools.

The campaign against the academies could involve strikes to stop a new employer taking over a school, but the "free" schools would be opposed through the use of planning legislation. In Sweden, the government has allowed them to be set up in office blocks and town halls and be run by parents' groups.

However, it is the proposed boycott of national curriculum tests which will pose the most immediate threat to a new government. Both unions are still balloting their members but are confident of a "yes" vote for the action. Christine Blower, general secretary of the NUT said: "I want to stress this is not a strike. Children will still be at their desks and in many, many schools may be able to do something more interesting than the tests."

The unions are confident they could survive any legal challenge to their action by claiming that the tests cause increased workload and stress for their members. The results of the tests are crucial in compiling primary school league tables, so staff are under pressure to "teach to the tests" in the final year of primary schooling, the unions argue.

None of the three major parties would scrap the tests this year. The Schools Secretary, Ed Balls, has hinted they could be replaced by teachers' own assessments of their children in future if their marking is proved to be as robust as the external tests.

Mr Gove has insisted that the tests and league tables are "here to stay" – although he has floated the idea of transferring them to the first year of secondary schooling.