The two biggest teachers' unions are split over the issue of boycotting national curriculum tests.

Speaking at his union's conference in Eastbourne, Eamonn O'Kane, general secretary of the National Association of Schoolmasters Union of Women Teachers, said the decision by the National Union of Teachers to ballot members on a strike was "stupid" and its boycott would probably be illegal.

"What exactly are they planning to boycott anyway?" he asked.

"The tests for 11 and 14-year-olds are set externally and delivered to schools for head teachers to administer and are then marked externally.

"Does the NUT plan to boycott the teaching which prepares pupils for the tests, in which case it would be a very serious development if they refuse to carry out their contractual obligations to teach children?"

At the NUT's conference in Harrogate, delegates also threatened to strike over the use of unqualified classroom assistants to take lessons and to walk out if asked to teach classes over 26.