A letter from Gillian Shephard, Secretary of State for Employment, to John Patten, Secretary of State for Education, suggests making it illegal for any trade unionist to take action 'clearly designed to frustrate the carrying out of a specific statutory duty'.
The letter, leaked to the National Union of Teachers, was written on 26 February - several weeks before the National Association of Schoolmasters/ Union of Women Teachers began its boycott of preparations for this summer's tests.
Union leaders reacted angrily, predicting that the move would strengthen teachers' resolve over the boycott. Doug McAvoy, NUT general secretary, said: 'Educational change brought about by force, and without the professional support of teachers, is unlikely to be effective in our schools.'
Although officials refused to comment, ministers conspicuously avoided ruling out the threat of legislation.
Mrs Shephard told Mr Patten that the law could be changed quickly, by introducing an amendment to the Trade Union Reform and Employment Rights Bill. She suggested removing trade union immunity 'where the principal or sole demand is that workers should not be required to do work which is necessary in order to carry out a specific statutory duty on the employer'. Unions would be free to take action over a pay rise they are demanding for carrying out statutory duties.
Mrs Shephard said 'it would mean that the parents of a child denied a test as a result of an unlawful boycott would be able to seek an injunction to get the boycott removed'. If the union failed to call off the boycott, it would be in contempt of court, and could be fined, or have funds sequestered.
Mrs Shephard rejected withdrawing immunity from any industrial action which interfered with the performance of statutory duties: that 'would be attacked by our opponents as tantamount to the removal of the right-to-strike from public sector employees'.
However, the employment department's approach is not the only option being considered. Education ministers could introduce an amendment to the Education Bill, which this week began its committee stage in the Lords, confining the restriction to teachers.
Both those approaches might just take effect before the tests for 14-year- olds in mathematics, science, English and technology, scheduled for early June. A third route might be legislation in the autumn.
The timing is governed by legal action between the London borough of Wandsworth and the NASUWT. The Court of Appeal will today give its judgment on whether the High Court was right to rule that the boycott is legal. Whatever the outcome, the case is likely to go to the House of Lords.
If the boyott's legality is upheld, ministers will move to change the law.