Teachers' and headteachers' leaders are expected to confirm next week whether they are to go ahead with balloting members over a boycott of the key stage 2 Sats tests.
Teaching unions the NUT and the NAHT are set to make the final decision next week, although it is not thought the vote will take place until April.
Unions are hoping any action this May will cause maximum embarrassment for the Government close to the general election.
Next week's decision comes as the NAHT has released detailed figures on its recent consultation with members regarding a boycott.
More than 40 per cent of headteachers responded, and 64 per cent of those said they were in favour of taking action. Eighty-five per cent said they supported a phasing out of the tests for 10 and 11-year-olds.
The association has already presented ministers with its detailed vision for the future of assessment, which proposes a move towards internal teacher-led approach. "Rotational sampling", where 30 per cent of pupils nationwide would sit external tests, would allow the Government to gauge the progress of the country as a whole.
Mick Brookes, general secretary of the NAHT, said the result of the consultation of 22,000 members was "extremely important" for its assessment reform campaign.
"It gives us greater determination to effect real change in 2010," he added. "We are still hopeful that we will be able to reach a negotiated way forward, but more needs to be done to reach this position."
Although heads voted in favour of a boycott at last year's annual conference, it is uncertain how many will actually take part, after the Government warned it would be "unlawful".
The NAHT is unlikely to go ahead with action without the NUT, whose own indicative ballot has thrown some doubt on teachers' enthusiasm for a boycott.
Only a quarter of members replied, of which 75 per cent backed a boycott, but leaders at the union said that it amounted to an "overwhelming rejection of the Sats regime".
Sources within the union say they do not expect the outcome of the indicative ballot to put the union off forging ahead.
Ed Balls, Schools Secretary, has already announced reforms to next year's tests, which will see teacher assessments published alongside the externally marked tests.
If the move proves popular with parents, the tests could be scrapped completely by 2011. However, Mr Brookes said he would not support anything that included league tables.
But this tension will be nothing compared with the conflict that would arise from a Tory win at the next general election.
Michael Gove, shadow education secretary, has pledged to keep national tests and league tables, claiming they give "robust data on the performance of schools".