Teachers are set to be balloted over a boycott of primary school Sats for the second year in a row, The TES has learnt.
The NUT is discussing plans to ballot its members within the next month in order to stop schools from ordering the test papers later in the autumn term.
The National Association of Head Teachers, which co-ordinated this year's boycott with the NUT, has written to heads this week, telling them to expect a decision over whether there will be a repeat ballot by "the end of the month".
Sources within both unions said they have not ruled out taking unilateral action if the other decides against a ballot to boycott the 2011 Sats.
This year, about 4,000 schools - 26 per cent of those eligible - took part in the boycott, causing considerable disruption to the administration of the controversial tests.
Unions believe the take-up could be higher if a boycott goes ahead next year as the decision will be taken before pupils have been prepared to take the exams.
Russell Hobby, general secretary of the NAHT, said Sats were still "loathed" across the profession and he expected any boycott to be well- supported. "We will not exit September without a firm decision about what we are going to do," he said.
An NUT source said: "There are discussions over a boycott going on at all levels of the union and we're confident there will be a decision to ballot soon. A lot depends on what the NAHT does but there will be pressure from some parts of the union to take unilateral action even if they don't take part."
Mr Hobby said it was possible that some sort of agreement could be reached with the Government - although indications from those close to negotiations suggested this is unlikely.
Mr Hobby urged Education Secretary Michael Gove to be as swift as possible in announcing his plans for accountability and testing in schools. "There's an opportunity for them there, we would listen if they wish to clarify their position," Mr Hobby said.
Mr Gove has already announced a review of the testing regime, acknowledging the current system has "flaws" and suggested he would be looking abroad for ideas on how to overhaul it.
In a statement in July, when Key Stage 2 results were published, he said: "I believe headteachers and teachers will agree the solution to strengthening the tests is to work together and agree on how they can be reformed."
The news guarantees the row over Sats will remain in the spotlight this year as the NUT joins other classroom teacher unions in ramping up opposition to Michael Gove's new academy expansion and "free schools" project.
Christine Blower, general secretary of the NUT, said no final decisions would be made on a ballot until after the Government's review.
"We are still pressing the Government for a proper review of the assessment and accountability regime and any response will be made in accordance with the outcome of the review," she said.
Both unions have called for an end to schools being compared through league tables, and want increased emphasis on externally moderated teacher assessment.
`DON'T WAIT FOR PERMISSION'
The former head of the National Association of Head Teachers has said schools should not wait for the unions to give the go-ahead over a Sats boycott, and should hold their own ballots in school instead.
Mick Brookes said schools should not "wait for permission" to opt out of the tests in May this year.
He said waiting around for unions to decide on what they are doing could lead to the same delays as last year, which meant schools only knew the action was going ahead at the last minute.
Mr Brookes told The TES he was not advocating schools "breaking the law", but they should hold legal ballots on a school level to send out a message to Education Secretary Michael Gove. "Don't wait for the NAHT or anyone else for that matter," he said. "Don't wait for a ballot on the eve of Sats."