The result of the ballot on a proposed boycott of key stage 2 Sats tests is expected today, potentially bringing unions a step closer to industrial action just days after the general election outcome is known.
Unions are expected to announce whether their ballot of 24,699 primary school leaders has achieved the majority "Yes" vote required to go ahead with plans to disrupt the tests.
But the final decision of the NUT and heads' union the NAHT on whether to go ahead with the action - which will take account of overall turnout - will not be announced until next Wednesday.
It will then be down to individual headteachers whether to toe the unions' line when the first exam takes place on May 10.
A poll by The TES earlier this month revealed that 51 per cent of teachers backed the boycott.
But some heads said they would administer the tests whatever the unions' decision because so much energy had already been expended preparing for them.
There are also concerns that the action could lack impact, given that it will start just two working days after the general election.
Mick Brookes, general secretary of the NAHT, said heads planning to defy a boycott were "missing the point". He claims that if only 2,000 out of 16,000 primary schools with Year 6 pupils went ahead with a boycott it would be enough to frustrate the compilation of the league tables.
He added that the unions had been "meticulous" in the organisation of the ballot.
They are determined to avoid the fate of Unite's British Airways cabin crew members whose strike was called off after irregularities were found in the ballot procedure.
A planned strike of railworkers by the RMT union was also halted after Network Rail was granted an injunction.
Although distracted by the general election campaign, the Government will still be keen to thwart any action, which Schools Secretary Ed Balls has already claimed would be "unlawful".
In March, Mr Balls made his clearest attempt to reduce backing for a boycott by writing to all primary headteachers to defend the tests.
He said the exams for 11-year-olds were a key measure of performance, but offered a glimmer of hope to the anti-Sats movement, insisting that the current testing regime was "not set in stone".
The National Governors' Association has already warned governing bodies "not to sign any statements" in support of the boycott, but has also warned them not to force the tests to be carried out.