Half of England's primary schools will boycott national tests due to be taken by 11-year-olds in just over a week, teaching unions claim.
National Union of Teachers general secretary Christine Blower was addressing members of the National Association of Head Teachers.
She said 50% of England's 17,000 schools would take part.
Results from the national tests - in English and maths - are used to make up the primary school league tables.
They also underpin reports by Ofsted inspectors.
Ms Blower said the unions could succeed in their boycott and that the two unions should find ways to boycott Ofsted inspections next.
The two unions have pitted themselves against the government in opposing what they say is the damaging effect of the national tests on children's education and schools.
They are particularly angry about the publication of the results in league tables, which they say humiliates and demoralises schools and do not reflect their true achievements.
Ms Blower told the annual conference of the NAHT: "We can succeed in the Sats boycott.
"All around the country, we are hearing 50% of schools and counting are going to do the boycott."
Head teachers are coming under pressure from local authorities and some governing bodies, who say pay can be docked from heads who do not carry out their duties and that other people could be brought in to run the tests.
The National Governors' Association says the situation is very confused and that governors are caught in a dispute between the government and head teachers.
Ms Blower told delegates: "The level of work caused by Ofsted and target setting is unreasonable and the Sats boycott is not unconnected with the pressure from Ofsted and the data-driven agenda".
Earlier, the head teachers had passed a motion calling for the replacement of the schools inspectorate Ofsted.
On Sunday, Schools Secretary Ed Balls will face the head teachers' conference and is expected to be given a tough time.
The heads have attacked the government for not listening to their calls to scrap the tests.
They will grill the top figures on education from the three main parties: Mr Balls, plus Michael Gove of the Conservatives and David Laws from the Liberal Democrats.
With days to go before the election and just over a week until the planned boycott, the heads will challenge the politicians on their plans for schools.
All three parties are against the boycott. Mr Balls says head teachers will be in breach of their statutory duties and that he is very disappointed that they are taking this action.
Both the Conservatives and the Liberal Democrats say they would reform the tests, with the Lib Dems saying they would make more use of teachers' assessments of children's abilities.
This year for the first time teachers' assessments of their children will be published alongside the test results for England.
When this was first announced, the government hoped it would be enough to head off a boycott of the tests.