The high-profile campaign to boycott this year's Year 6 Sats has split primary school teachers, with only half saying the protest should go ahead, a TES poll has found.
Evidence of a divide in the profession comes as the issue is thrust back in the limelight at the annual conferences of the two largest teaching unions this weekend.
The NUT will use its gathering in Liverpool to ramp up pressure for its proposed joint action with the NAHT next month.
But the NASUWT, which is meeting in Birmingham, will warn that replacing Sats with teacher assessment risks overloading staff with extra work.
Despite widespread dissatisfaction with Sats, just 51 per cent of 500 primary teachers polled by The TES said they backed a boycott of this year's tests. Forty-one per cent are against the action, with the rest undecided.
The figures come as the NUT and the NAHT reach the halfway point in their ballot of 24,699 primary heads, deputies and assistant heads over whether to disrupt the administration of the tests. Both the unions say they are confident their members will back action.
However, some observers have claimed large numbers will only vote in favour of the boycott because a "no vote" could be seen as tacit support for Sats themselves.
Many schools are already beginning to plan for a boycott, with consortia of primaries trying to reach agreement on how to handle the situation.
Some schools have already decided that pupils will sit last year's papers so as not to disappoint children who have prepared and parents who expect results.
But other schools are expected to defy the potential boycott, even if their leadership teams are members of the NUT and NAHT.
NAHT general secretary Mick Brookes said that heads who argued they "might as well" go ahead with the tests because the children had trained for them "completely missed the point". Results are used to "humiliate and demean" schools through Ofsted and league tables, he said.
Judith Kearney, 65, a Year 6 teacher at Summerhill Primary in Liverpool, said she supported the boycott. "There's strong pressure to do well in the Sats, but I know that the secondary schools don't take any notice of them," she said.
But Huw Thomas, head of Emmaus Primary in Sheffield, said he was against this boycott because it had "come too late".