Thurrock Headteachers have announced plans to boycott next month’s Sats tests for primary school pupils.
It throws into confusion the national tests due to be taken by hundreds of thousands of 10 and 11 year olds.
Mick Brookes, general secretary of the National Association of Head Teachers, told the BBC that the boycott is against a “flawed testing regime”.
Schools Secretary Ed Balls urged heads to “think hard before disrupting children’s learning”.
Mr Balls said there was a statutory duty on heads to oversee tests – and that he would issue advice to governors and local authorities on the threat of a boycott.
Head teachers say they will not administer tests which they say are “misused to compile meaningless league tables, which only serve to humiliate and demean children”.
But they say the boycott is “categorically not strike action” – and that schools will remain open when tests are due to be taken, from May 10 to 13.
Instead of taking tests, this will be a “brilliant week, a creative week”, says Christine Blower, leader of the National Union of Teachers, which is also taking part in the boycott.
Mr Balls said it was “extremely disappointing that the NAHT and NUT are pressing on with their dispute when the clear majority of heads and deputy heads do not back this action – over two-thirds of their own members did not vote to support disrupting tests”.
“Pupils and teachers have been working hard all year and they should all be given the opportunity to demonstrate their achievements. It would be very unfair if some children were prevented from doing so at the last minute.”
Conservative schools spokesman Michael Gove said: “The national curriculum tests need to be reformed but not scrapped.”