School governors should consider telling heads to stay away from their own schools if they plan to take part in the key stage 2 Sats boycott, to allow other people to administer the tests, the Schools Secretary has said.
In a letter to governing bodies, Ed Balls said they should consider instructing headteachers to "remain absent" from school premises at times when tests are due to take place, so that "another competent person" could see they go ahead.
The statement raises the possibility that governors will bring in extra staff willing to do the work, as Mr Balls admitted staff with union membership cannot be expected to take on extra duties their heads are refusing to carry out.
Mick Brookes, general secretary of the NAHT, warned that Mr Balls' advice was "inflammatory" and could cause lasting damage to working relationships.
In his strongest published missive on the boycott so far, Mr Balls told governors that heads had a "professional and moral duty to pupils and parents" to administer the exams.
He said he hoped they would "think before disrupting children's learning, confusing and inconveniencing parents and damaging the profession's reputation".
Reminding governors that they have a "statutory duty" to ensure the tests take place, he added that they should inform the Qualifications and Curriculum Development Agency assessments hotline if their school is not running the tests.
Christine Blower, general secretary of the NUT, said it was "revealing" that the letter did not mention the industrial action was illegal.
The letter comes just under a week before the NUT and NAHT's boycott of the Sats is due to start on May 4.
The unions have asked any schools planning to participate to leave boxes of test papers unopened, if they are delivered on or after that date.
Schools which want to take part and receive papers before that date will still be legally obliged to open and check the papers, as they will not have the protection of a trade dispute.
The refusal to open boxes of papers will mark the kick-off of the first industrial action a new government will have to face, whatever its party hue or hues. The tests begin on May 10, four days after the election.
Unions say their research shows that between 4,000 and 5,000 schools with Year 6 pupils could take part in the boycott, but a majority of a total 16,000 schools are expected to go ahead with the tests.
A poll of teachers by The TES a month ago revealed a split in the teaching profession over the boycott, with 51 per cent in favour of action.
Union leaders say it would only take 2,000 schools to opt out of the tests this year to disrupt the compilation of "league tables", which they say put "intolerable pressure" on headteachers.
Many heads have expressed concern that the industrial action is badly timed so close to the election, and will leave children who have revised hard "disappointed".
There are concerns, too, over demanding parents who want to see concrete evidence of how their children are doing at school.
Mr Balls has already written to headteachers, explaining that the Sats tests are not "set in stone".
But the unions see the decision to disrupt the administration of the tests as the only way to convince the Government to put an end to external tests for 11-year-olds.
Emma Knights, chief executive of the National Governors' Association, said she was "not convinced" governing bodies could be exposed to legal action if they ordered heads from the premises and brought in alternative staff.
She said: "We don't agree with the department's interpretation of what governors can do under the law," she said.