Heads who put pressure on teachers to boycott next year's primary school Sats will face disciplinary action, governors have warned.
The National Association of Head Teachers and the NUT revealed last week that they will give members a vote on boycotting key stage 1 and 2 tests in a serious escalation of their campaign to abolish the exams.
But Phil Revell, chief executive of the National Governors' Association, said heads risked putting their staff and governing bodies in a difficult position that could force governors to take disciplinary action.
"There is absolutely no way that pressure should be brought on any teacher who wishes to carry out their lawful duties and administer the tests," he said.
"Governors will make it absolutely clear that undue pressure brought on colleagues will result in disciplinary action. We really regret that headteachers are going down the road of a boycott."
Mr Revell said he opposed headteacher involvement in the proposed boycott as it could compromise their position as a school leader. "Being a senior manager means that in certain situations you have to put your personal misgivings to one side and do the job you are paid for," he said. "It is completely different for a head, as opposed to a teacher, to take this kind of action."
Headteachers have a statutory duty to administer the key stage tests. Mr Revell said governing bodies could not "pick and choose" the legislation they implement.
Mick Brookes, general secretary of the NAHT, said the union had taken legal advice and was confident that a boycott could be justified as a legitimate trade dispute.
The governors' association is still taking advice on what options are open should the boycott go ahead. Mr Revell said he expected that disciplinary action would be available even if pressure was not put on staff.
However, he admitted that many governors would be reluctant to take that step as it would not be "good industrial relations". Governors may wish to avoid confrontation for the benefit of the long-term future of the school, he said.
Other teaching unions have criticised the NUT and NAHT, saying their action will make the Government less likely to drop the tests.
Chris Keates, general secretary of the NASUWT, said staff opposed to the boycott would be put in a "real dilemma". "We will have to say they could be in difficulties if they go against the instructions of the headteacher," she said.
Mary Bousted, general secretary of the Association of Teachers and Lecturers, said it was too early to say what advice it would issue.
Around 80 per cent of primary school heads are NAHT members, which appears to put the union in a strong position if the proposal is passed at its conference in May.
But Dr Bousted questioned this week whether the industrial action was "deliverable".
"It is one thing for the NUT executive and the NAHT council to get worked up about this; it is quite another for NAHT members to take what will be an unlawful action, to refuse to administer and give the results. While the NAHT council might be gung-ho, I don't think the members will be."
Mr Brookes said: "The NAHT and the NUT can make their minds up themselves and I don't think they need help from ATL or anyone else."
An NUT spokeswoman said: "There is no question of NUT or NAHT taking action that is unlawful. We clearly wouldn't do that - it would be lawful action."
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