Primary teachers' careers put are being at risk by heads who force them to invigilate Sats, a union has warned.
The problem of classroom staff having to invigilate national curriculum tests is widespread in primaries, despite the duty being removed from teachers' contracts four years ago, the NASUWT said.
Teachers come under pressure from heads to oversee tests when schools should be employing other staff to do it, the union says.
As well as adding to teachers' workload, concerns have been raised that teachers who invigilate are at risk of allegations of professional misconduct because of the temptation to help pupils.
The union is due to launch a high-profile campaign this week to crack down on invigilation by teachers.
Sue Rogers, the union's national treasurer and chair of the AQA exam board, said at the union's annual conference in Bournemouth last week: "Primary schools are rife with the invigilation of Sats and it should not be happening. You lay yourself open professionally to real risk if you are seen to be invigilating Sats of your own class and your own group.
"I have sat too many times listening to examples of teachers just thinking they were helping children and thereby ruining their career," she said.
Kathy Duggan, a primary teacher and NASUWT representative for Lewisham, south London, told the conference: "The majority of primary school heads over the years have actively encouraged, insisted and forced our members to break the law and invigilate the Year 6 Sats.
Teachers were "morally blackmailed" into doing it, she said.
Ms Duggan said teachers were "shamelessly" made to encourage pupils to cram for tests. She also warned that if key stage 2 Sats were replaced with teacher assessment, heads would have "absolutely no hesitation in replacing bullying of staff over Sats with bullying members with continuous, pointless, excessive assessment regimes".
Mick Brookes, general secretary of the National Association of Head Teachers, hit back, saying classroom teachers were free to volunteer to invigilate and often wanted to do so.
"It is nothing to do with teachers being forced," he said. "Teachers choose to do it because of the immense pressure on schools to perform for league tables.
"They know that if children are being watched over by someone they don't know, it can have a deleterious impact on their performance."
Mr Brookes said that paying invigilators to cover KS2 tests costs schools about Pounds 1,500.
Chris Keates, general secretary of the NASUWT, agreed that some teachers wanted to invigilate, but said, as well as allegations of exam malpractice, it could lead to teachers facing General Teaching Council charges.
Figures released by the Qualifications and Curriculum Authority show that there were 252 reported cases of maladministration related to last year's KS2 tests. The most common allegations, accounting for 18 per cent of cases, were of test administrators "over-aiding" pupils.
The General Teaching Council for England has heard at least 12 cases against teachers involving allegations to do with Sats exams, although the specific figure relating to invigilation was unclear at the time of going to press. At least two teachers have been suspended for their conduct relating to Sats.