Teachers overwhelmingly backed a boycott of Sats tests today, setting them on a collision course with a new Government.
The National Union of Teachers (NUT) and the National Association of Head Teachers (NAHT) have been balloting members involved in the administration of the tests - including headteachers, deputies and assistant heads - for the past month.
According to today's results, 74.9% of NUT members who voted were in favour of a boycott, as were 61.3% of NAHT members who voted.
Turnout for the NUT's ballot was 33.8% and for the NAHT it was 49.7%.
The executives of each union will now meet next week to take a final decision on whether to press ahead with the boycott.
But with large majorities in favour, the scene is now set for a showdown with whoever forms the next government.
If approved, the action will start on Tuesday May 4 - two days before the country goes to the polls.
Sats in English and maths are due to be taken by 600,000 11-year-olds in the week beginning May 10 - just days after the election.
NUT general secretary Christine Blower said: "This is an extremely creditable result which shows the resolve of our members to frustrate the administration of the Sats this year."
NAHT general secretary Mick Brookes said: "This is a significant result for the NAHT. We have not conducted a national ballot in a quarter of a century.
"This ballot and the impending action was entirely avoidable. Both the NAHT and NUT put forward a viable alternative for 2010 that would have produced a more accurate summary of a child's learning journey, would have reduced bureaucracy and would have saved the £23 million spent on this year's administrative arrangements."
Mr Brookes said he "took his hat off" to the "courage" of NAHT members in backing the boycott, as many are the sole person in their school responsible for administering the tests.
"I take my hat off to the courage of members who, despite quite heavy threats on this, have taken their courage in their hands and said 'Yes, we are prepared to take action on this because it is so important," he said.
Around 24,000 people were balloted between the two unions.
Mr Brookes said "many thousands" of primary schools could now be involved in a boycott. There are around 17,000 primaries in England.
The unions argue the tests are bad for children, teachers and education, and cause unnecessary stress. They also want to see school league tables abolished.
But Schools Minister Vernon Coaker urged heads not to support a boycott.
"Heads and teachers don't just have a statutory duty to make sure tests go ahead, but a professional responsibility to their pupils and their parents," he said.
"A boycott of this year's tests would not be in children's best interests. We urge NAHT and NUT executive members - and heads and deputies round the country - to think hard over the next few days and to decide not to disrupt children's testing and learning."
Ms Blower said the unions want the Government to introduce a national sampling system for maths and English - in which a selection of schools sit tests.
"A sampling system would give a national picture of pupil achievement without identifying individual schools or children," she said.
"Parents would still find out how their child is progressing. Reports to parents would come from teacher assessment, as is currently done in Wales, Scotland and Northern Ireland."
Speaking at the NUT's annual conference in Liverpool at Easter, Ms Blower said the boycott was a trade dispute focused on headteachers being undermined by the creation of league tables using Sats results.
She has called the boycott "industrial action with no downside" as teachers and pupils will still be at school, and it does not involve strikes.
Members will "frustrate the administration of the tests".
The two unions originally passed resolutions calling for a boycott at their annual conferences last year.
At that time the Department for Children, Schools and Families (DCSF) said headteachers have a statutory duty to administer the tests, and to not do so would be unlawful.
It is not yet known what action, if any, the Government will take to prevent the boycott.
John Dunford, general secretary of the Association of School and College Leaders (ASCL), acknowledged that the current testing system needs reviewing.
But he added: "There must remain some form of robust external assessment, alongside teacher assessment, so that secondary schools have a baseline for judging progress.
"Just as secondary schools are held to account through GCSE and A-level results, it is entirely right that there is some form of assessment at age 11 through which primary schools are held to account."
Margaret Morrissey, of campaign group Parents Outloud, said they were supportive of a boycott.
A poll of 1,333 parents on the group's website found that 97% were in favour of scrapping Sats, she said.