The National Union of Teachers will refuse to mark tests for seven-year-olds, due to start in 10 days' time, and tests for 11- and 14-year-olds next term.
The decision will bring confusion to schools because the two other main teachers' unions have agreed to mark the tests though not to carry out national curriculum assessments. Last year all three boycotted the tests as too cumbersome and time-consuming.
David Hart, general secretary of the National Association of Head Teachers, said the boycott would thwart the Government's aim of comparing schools because few would have reliable results.
A survey carried out for the NUT by the Electoral Reform Society and released yesterday showed that 90 per cent of its members support its campaign. It also showed that the vast majority of teachers believe the workload involved in the tests is excessive, despite government efforts to prune the curriculum and testing.
Doug McAvoy, the union's general secretary, said: 'Teachers want time to teach. They do no want to waste time on unnecessary and unsound statutory assessment and testing. The Government should now announce a moratorium for this year and next.'
The Department for Education said parents would not understand why the union had not called off the tests boycott in line with the other unions.