Head teachers planning to boycott national tests for 10 and 11-year-olds in England say they are being threatened with disciplinary action and docked pay.
The National Association of Head Teachers and some members of the National Union of Teachers voted to boycott the Sats, which take place in a week.
The heads say about half of primary schools will take action, but the picture is far from clear.
School governors, meanwhile, say they are being asked to "make decisions which could inflame the situation".
They say they are caught in the middle of a dispute between the government and heads.
Schools Secretary Ed Balls wrote to governors on Wednesday setting out their responsibilities and what action they could take if their head joined the boycott.
It includes considering sending heads home and bringing someone else in to run the tests.
The National Governors' Association says it could be illegal to send heads home and is asking the government for more guidance.
Clare Collins, from the NGA, says there is a "huge climate of uncertainty", particularly around the legal position of the dispute.
"We don't have clear information and we are concerned about confusion in schools about what is happening," she said.
"We are being asked to make decisions which could inflame the situation. We are caught between the government and the head teachers."
Results from the national tests - in English and maths - are used to make up the primary school league tables and underpin reports by Ofsted inspectors.
The heads and deputies have not been mandated by their unions to take the action, so will not be thrown out of the organisations if they decide not to boycott the tests.
Members of the NAHT union are meeting for their annual conference in Liverpool.
One of the delegates, Leeds head teacher Stephen Watkins, says heads and governing bodies are coming under pressure.
He said the city's education authority had sent letters to governors - copied to heads - saying they could consider disciplinary action.
The letter also told governors they had a legal duty to make sure Sats were carried out and that could include asking volunteers to administer them.
"Most governing bodies are supporting head teachers, they will not act against head teachers," he said.
The tests are not going to be taken at Mr Watkins' school.
Another head teacher, Roy Tedscoe, said Hackney Learning Trust had said it would dock the pay of everyone boycotting the tests for every day.
And heads were being told they could expect a visit from someone "high up in the trust" if they took part.
The NAHT's in-coming president, Mike Welsh, said: "Members of local authorities do not want to take action but are being pressured to send out these letters and make these comments."
Another head, who asked not to be named, said his governing body had told him he could face disciplinary action.
"My governing body believes children should be tested. My chair of governors is also using emotional abuse, saying 'do I wish this to be my legacy - to be remembered as the head who boycotted Sats'," he said.
He said the test papers had arrived at his school but he had sealed the box back up and hidden it.
In their joint letter to school governors, Mr Balls and Schools Minister Vernon Coaker said they were "deeply disappointed" by the union action.
They said it was "unacceptable to deny parents a full picture of the progress their child was making and information about their local school".
Government lawyers have been examining grounds for a possible legal challenge, which the NGA believes is unlikely, particularly with the election looming.
Heads have faced criticism from parents about pulling the plug at this late stage, when children have been working towards the tests.
NAHT general secretary Mick Brookes apologised to parents, but said the rules governing industrial action meant action had to follow within four weeks of a ballot.
Heads argue the tests distort the curriculum because teachers have to focus so much on them.
They say the league tables compiled from the results are damaging and do not reflect the true achievements of schools or children.
The Conservatives and Liberal Democrats, like Labour, are against the boycott. Both say they would keep the tests but reform them.