Would the courts be embroiled again?

The tests were last boycotted in 1994.

On that occasion, the boycott was started by the National Association of Schoolmasters Union of Women Teachers.

The union successfully defended a challenge to its legality by Wandsworth Council in the High Court.

The council argued that the action was illegal under union law as it was not a valid trade dispute.

But the court ruled that the extra workload associated with the tests was such that the NASUWT was within its rights to take industrial action and refuse to administer them.

So all three big unions mounted a boycott. The existing tests were slimmed down and planned extra tests in other subjects never materialised.

Judgement

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This time, the National Union of Teachers says the advice from its legal counsel if that there is again a valid industrial dispute.

A spokesperson said it would be about teachers' working conditions and their professional judgement being undermined.

Any legal challenge would be a matter for the teachers' employers - technically, local education authorities in most cases.

"They have a general duty to teach and apply the national curriculum and testing is included in that," a spokesperson for the Department for Education and Skills said.

She said the matter would be reviewed if a boycott did go ahead.

Clash

The union's decision puts it on a collision course with ministers for the second time this year.

Alone of the major education unions, the NUT has refused to sign the workload agreement intended to reduce the burden on teachers.

Although welcoming its aims, the union believes it opens the way for unqualified staff to be used in place of fully-qualified teachers.