The letter, sent to heads and chairs of governing bodies on the eve of a court case which will decide whether a teachers' boycott of this summer's tests is legal, does not detail what heads and governors should do, but says: 'It will be your responsibility to make it clear that you expect the teachers in your school to fulfill their professional and contractual duty to administer the tests.'
The Association of Teachers and Lecturers yesterday began balloting more than 98,000 members in state schools on a testing boycott. The National Association of Schoolmasters Union of Women Teachers has already voted for a boycott and the National Union of Teachers is also balloting members.
As the battle between Mr Patten and the teacher unions gathers pace, the extent to which governors will be held legally liable for the effects of the boycott is being debated.
Even if Wandsworth council loses its case in the Court of Appeal against the NASUWT, which begins today, the boycott will face a fresh legal challenge. A right-wing pressure group, the Campaign for Real Education, yesterday repeated its threat to take to court heads and governors who say they support the boycott.
Heads and governors, however, believe that the success of legal action against governing bodies is far from certain. Walter Ulrich, of the National Association of Governors and Managers, said the legal situation was unclear: 'The key fact is that governors can't make these tests happen if teachers won't conduct them.'
David Hart, general secretary of the National Association of Head Teachers, said: 'I don't think governors should worry . . . To talk about taking governors to court when they can only operate through the agency of the head and with the co-operation of the teachers is to misunderstand how schools work.
'No amount of sabre rattling by the Government will make a blind bit of difference to the boycott.'
Governors might be vulnerable to an action by parents, but even then a claim for damages would be difficult to prosecute. It would be necessary to show that children's education had been damaged because they had not taken the tests or to claim money paid to outside teachers brought in to conduct the tests.
The biggest problem for a governing body facing such a challenge would be finding the money to cover legal costs.
What can governors do in response to Mr Patten's warning? Teachers in most schools are employed by local authorities, not governors, so, as Mr Hart pointed out, councillors are responsible for docking pay. Only governors in voluntary-aided schools and those which have opted out of local authority control employ teachers.
The Department for Education would not say yesterday whether Mr Patten expected heads and governors to take disciplinary action against teachers. His letter asks them simply to give 'a clear lead'. Unless the boycott is declared illegal by the courts, even that seems unlikely.
Mr Patten would have little to gain from a court case against governors. It would raise the political temperature and threaten the Government's policy that schools should manage their own budgets. That policy depends on thousands of governors volunteering to manage schools.