By Gary Eason
Education editor, BBC News website
The proposed boycott of national curriculum, or Sats, tests in England by two of the education unions - which the government says would be unlawful - is not without precedent.
It is not yet clear how a boycott would be organised by the unions
There was a boycott in the early 1990s - and a legal challenge to it failed.
Then, the boycott was started by the National Association of Schoolmasters Union of Women Teachers (NASUWT).
The union successfully defended a challenge to its legality, mounted 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 - with the National Union of Teachers (NUT) and the Association of Teachers and Lecturers (ATL) joining in - mounted a boycott.
The existing tests were slimmed down and planned extra tests in other subjects never materialised.
Then in 2002 the NUT began a fresh campaign against the tests.
A survey of members indicated widespread opposition to them, and a ballot on a boycott was organised.
It overwhelmingly backed the proposal - but not by a large enough number of the members, under the union's rules, for the boycott to be sanctioned.
At least 51,866 votes were needed - but only 30,452 were forthcoming. Another 4,875 members voted against. Only about a third of ballot papers were returned.
The NUT's executive council is now trying again to organise a boycott - along with the leadership of the National Association of Head Teachers (NAHT), which claims to represent 85% of primary school heads.
The government has said that this would be unlawful.
It cites section 87(3) of the Education Act 2002 (establishment of the National Curriculum for England by order) which gives the secretary of state power to make orders specifying attainment targets, programmes of study and assessment arrangements in relation to the key stages.
The power to make orders includes power to "confer or impose functions on" (amongst other people) head teachers.
It is not yet clear how, if it came to it, the unions would organise a boycott.
But three years ago the new general secretary of the NAHT, Mick Brookes, suggested schools could disrupt pupil tests - by asking parents to send their children in late.
If less than two-thirds of pupils did tests, results would be invalid, he said.
In the event his move to disrupt the testing came to nothing at the time because, he said later, his members around the country would not support it then.
But this raises the possibility that the future of testing would depend on whether parents really do dislike it - or whether, as the government says its research shows, parents want the tests to continue.