Decision-makers at teaching union the NUT were this week considering the results of an indicative ballot largely expected to be in favour of a national boycott on Sats tests.
The union is expected to announce that its members have shown strong support for refusing to administer the Year 6 tests in May next year.
The NUT ballot follows a consultation last week by the National Association of Head Teachers (NAHT) indicating the majority of heads would also back action this spring.
Well-placed sources told The TES the NUT's own poll of members would reflect that of the heads.
A final decision on whether to hold a full-scale ballot will be made by the two unions in January.
The unions, representing a large proportion of England's primary teachers and the majority of primary heads, have teamed up to campaign for the abolition of Sats and the introduction of a new system of teacher assessment.
They say they will only take action if "pushed to the brink" by a Government bent on maintaining the status quo.
The two unions have already tested the strength of feeling among members this year at their annual conferences, where both passed motions supporting a boycott.
Last week, the NAHT launched its own vision for the future of testing in primaries, based on teacher-led assessment backed up by a scheme of chartered assessors, school improvement partners and Ofsted.
They said "rotational sampling", where 30 per cent of pupils nationwide would sit external tests in English, maths and science, would allow the Government to follow the progress of schools as a whole.
Although a vote over a boycott looks likely, Schools Secretary Ed Balls has already made three key concessions on Sats in the space of a year, indicating a degree of flexibility on the issue.
First, testing at key stage 3 was abandoned, then Mr Balls agreed to drop science tests at Year 6. In a more recent announcement, he said that the results of teacher assessment would be published alongside external exam results from 2011.
Mick Brookes, NAHT general secretary, said this did not go far enough, but Mr Balls has said his department is "always willing to change".
Public support for a boycott has so far been mixed. In May this year, the National Confederation of Parent Teacher Associations found that just 41 per cent of 2,200 parents surveyed gave their support.
However, a petition calling for the Government to scrap the tests, signed by celebrities, teachers, educationists, parents and members of the public has reached 10,000 signatures.