Teachers and heads across England will begin receiving ballot papers this week as a campaign to scrap Sats tests gathers pace.
The National Union of Teachers (NUT) and the National Association of Headteachers (NAHT) are holding an indicative ballot of members over proposals to boycott next year's tests.
At a conference later today the two unions will give details of the next stage of their campaign to end the tests.
The two unions passed resolutions at their annual conferences earlier this year, proposing to take joint action to boycott the tests if they are not scrapped.
Between them, the two unions represent a large proportion of teaching staff in primary schools.
Under the indicative ballot, members will be asked if they would show support for a boycott if a solution is not found. A full ballot would still be taken at a later date.
NUT general secretary Christine Blower said: "This is our opportunity to find out how members really do feel about the Sats."
She added: "Whenever I bump into primary teachers they say 'I really hope there aren't going to be Sats next year."'
She said the campaign was not about schools and teachers not being held accountable, but making sure they are accountable on a proper basis.
The two unions want to see Sats replaced by teacher assessment.
They argue the tests are bad for children, teachers and education, and cause unnecessary stress.
A petition against the tests, set up by the two unions, already has tens of thousands of signatures.
On Friday Gordon Brown said Sats tests are as important as GCSEs or A-levels in holding schools to account.
Writing in the Times Educational Supplement (TES) the Prime Minister said: "I'm not willing to accept excuses for underperformance.
"Every schools should be doing the best by all its pupils. But progress relies on the need to retain clear accountability through testing.
"This means at the end of primary school as much as at the end of secondary."
Schools Minister Vernon Coaker said: "A boycott of statutory tests would be disruptive to pupils and risk doing real damage to the standing of the profession. It is out of step with what parents want; not backed by other teaching unions; and would be a clear breach of heads' statutory duties if the tests do not go ahead.
"NUT and NAHT should reflect on all of our reforms before continuing with their proposed action - replacing externally marked Key Stage 3 tests and Key Stage 2 science with teacher assessment; trialling when ready testing; and piloting report cards to give a much broader picture of schools' performance.
"It's time to challenge the myth that children spend all of their school days preparing for the tests - from next year, children will do just two externally marked tests before they are 14. We've made it repeatedly clear that drilling with practice test papers is wrong. Excellent teaching throughout the year is improving pupils' reading, writing and maths skills and is the best preparation for tests.
"Our expert group on assessment concluded that scrapping externally marked English and maths tests was the wrong approach. It categorically said current English and maths tests are educationally beneficial; vital for public accountability; and a key part of giving parents objective information on their children's progress after seven years in publicly funded education."