Industrial action over Sats tests is expected to move a step closer today as a headteachers' union prepares to ask its members to back a proposed boycott.
At their annual conference in Brighton today the National Association of Head Teachers (NAHT) will put forward a resolution calling for a ballot on industrial action.
The motion, which is widely expected to be approved, could mean heads and teachers refuse to prepare or invigilate for the exams next year.
The Government has insisted that any such industrial action would be unlawful.
The Department for Children, Schools and Families (DCSF) has announced that its lawyers have said headteachers have a statutory duty to administer the tests, and to not do so would be unlawful.
Today's resolution comes just weeks after the National Union of Teachers (NUT) unanimously passed an identical motion at its annual conference in Cardiff.
Schools secretary Ed Balls will address the NAHT conference ahead of the Sats resolution later this afternoon, and is likely to receive a mixed reception from delegates.
His speech comes just a week after he faced intense pressure from MPs over accusations from Dr Ken Boston, former chief executive of the Qualifications and Curriculum Authority (QCA), that ministers misled Parliament and an independent inquiry into last summer's Sats fiasco.
NAHT general secretary Mick Brookes said today that he would like Mr Balls to come in and admit he is making a mistake in keeping Sats tests for 11-year-olds.
In an article for the Times Educational Supplement (TES) yesterday, he said: "What is irresponsible about wanting to see an end to a system that has been proven to mar the final year of a child's primary experience?" The NAHT also called for the Contextual Value Added (CVA) measure - a system used to work out how much value a school has added to a child's education - to be reviewed.
It said that the way CVA is calculated is flawed and that it is "inconsistently" applied.
An expert group, set up by the Government last year to look at the tests - which are taken by 11-year-olds in English, maths and science, is due to report back in a few weeks.
Last month Schools Secretary Ed Balls said the current system was not set in stone and that he would be "surprised" if the expert group said it could not be improved.
The NUT and the NAHT announced in March that they plan to put an identical resolution before members calling for a boycott.
If today's resolution is passed, it instructs the unions' executive or national council to step up the joint campaign to halt testing.
As a last resort members should be balloted for joint action to boycott the tests. A ballot could then take place in the autumn term.
Industrial action would follow next year if the ballot was successful.