Headteachers are being threatened with the prospect of having their pay docked for taking part in next month's Sats boycott, union leaders warned today.
The National Association of Head Teachers (NAHT) claimed members are under mounting pressure from Government and local authorities to call off the industrial action.
Councils have been considering what action can be taken to stop the boycott, it emerged today. It comes after Schools Secretary Ed Balls wrote to governors earlier this week telling them they have a "statutory duty" to ensure the tests go ahead.
At the NAHT's annual conference in Liverpool today, union vice president Mike Welsh said councils are being leaned on by central government.
He said: "Across the country, we know local authorities did not want to take any action on this, and they themselves are being pressurised."
There is an "atmosphere" coming from the Department for Children, Schools and Families headquarters, Mr Welsh said, that is putting pressure on authorities to "send letters and make comments".
"They've been pressurised to do so, and they were quite happy to take a neutral stance," he said.
NAHT member Roy Tedscoe claimed that at a meeting of primary headteachers in Hackney, east London, yesterday Alan Wood, chief executive of the Learning Trust, which deals with education for the borough, said that heads who take part in the boycott will have their pay docked for every day they are involved in the action.
Heads were also told they will be expected to justify to the Learning Trust why they are boycotting the tests.
In a statement, the Learning Trust said a letter was sent to the chair of governors at each primary school in the borough yesterday asking them to remind senior staff of their duty to administer the tests.
"The letter advised that any refusal to make arrangements for the tests would be regarded as a breach of contract," the statement said.
"In line with advice provided at a national level by the Local Government Employers' organisation and at a local level by the London Councils' employers group, the Learning Trust has advised schools taking action that a notional amount of one day's pay can be deducted from staff salaries."
According to the Local Government Employers' advice, in the case of industrial action, including action short of a strike, "governing bodies are entitled to withhold pay for work not done".
The guidance adds: "However, the governing body who wishes to withhold pay will need to consider how much pay can be withheld in response to the 'partial performance' being offered by teachers."
Stephen Watkins, head of Mill Field Primary School in Leeds, said he knew of governors that had been sent a letter saying they should consider disciplining headteachers that take action, but they should refer it to the HR department before doing so.
He added that he believed governors are also being told to look for "alternative people" to administer the tests if necessary.
Chris Edwards, chief executive of Education Leeds, said: "We were approached by several governing bodies asking for advice about how to handle the possibility of a boycott. We have not advised governors to discipline headteachers if they do decide to boycott the Key Stage 2 Sats. What we have done is advise every governing body to talk to us before taking any action in the event of the tests not going ahead."
In his letter to governors Mr Balls said, if necessary, governors should look for another "competent person" to administer the tests, and should consider telling headteachers planning on taking part in the boycott to "remain absent from school" while the tests are taking place.
Mr Watkins said: "If my governors were to ask me not to go to work I personally would still go in, and if these people turned up I would not let them through the door without a CRB check, a passport, and a written letter instructing me to let them in, by which time (I) would have got the Year 6 children out of the back door."
Advice published yesterday by the National Governors Association says there is a "legal requirement" on governing bodies "to exercise their functions with a view to securing that the current assessment arrangements are implemented".
But in a seeming contradiction to Mr Balls's instructions, it adds: "If the industrial action is lawful (as the NGA suggests it has to be considered until there is case-law to draw on) the NGA would caution governing bodies against making alternative arrangements.
"If a headteacher decides to boycott the tests, the governing body should not get involved in operational matters and should not get involved in the administration of the tests."
NAHT general secretary Mick Brookes insisted that members will not be leaned on.
"We are not going to stand idly by, and watch our members threatened," he said.
He added: "We will, if people want to escalate this, then we will escalate. We think that's an extremely foolish thing to do, particularly at this point in time. This is a protest, it's a protest that will happen in the week 10-14 May and my advice to government would be, 'OK, you thought we wouldn't do it, let's have a look and see what happens now."'
The NAHT and the National Union of Teachers (NUT), who together represent around 80% of primary school heads, announced last week that the boycott will go ahead, after ballots of their leadership members showed support.
National Curriculum tests (known as Sats) in English and maths are due to be taken by 600,000 11-year-olds in the week beginning May 10 - just days after the election and on what is expected to be the first day of office for a new government.
It puts the unions on a collision course with a new government, whatever its colour.
Both Labour and the Conservatives have insisted that Sats should not be scrapped, although Mr Balls has said the system is "not set in stone", while the Tories have pledged to reform the tests.
The unions argue the tests are bad for children, teachers and education, cause unnecessary stress, and lead to the creation of league tables which undermine the work of schools and heads.
The Government has not yet ruled out taking legal action to stop the boycott.