Headteachers called for the national primary school testing system to be overhauled, as the Government prepared to publish this year's results.
Mick Brookes, general secretary of the National Association of Head Teachers (NAHT), warned that the tests in English, maths and science "distort" education for 11-year-olds.
Schools should be free to use teacher assessments to measure the progress of individual pupils, rather than being forced to push children through the tests, he said.
Ministers will publish figures showing how England's 11-year-olds performed in their National Curriculum tests.
Mr Brookes said: "The tests skew the curriculum. All the research we have is that the curriculum needs to be broader. We are letting the assessment system dominate the curriculum and it should be the other way round."
Mr Brookes said a child who falls short of Level 4 in English, for example, by a few marks should not be branded illiterate. He warned that some children find themselves spending much of their final year at primary school preparing for the tests.
"For some schools, particularly where they are struggling to raise standards, children do have their final year spoiled because they are spending so much time rehearsing the tests," he said. "I'm not blaming schools but we need to change the culture so that there is a greater emphasis on teacher assessment."
He said some testing is acceptable as a "check point" but the current system is too pressurised. Teachers are "demoralised" by the "political posturing" around testing and the league tables which rank schools on their performance, he said.
Last year, the Government missed its target to get 85% of 11-year-olds in England up to Level 4 - the standard expected of their age group - in English and maths by 2006. National Curriculum Key Stage 2 test results showed 79% of 11-year-olds in England reached the expected level in English and 76% in maths last year. For science, 87% reached Level 4.
Ministers are piloting the idea of testing individual pupils when they are ready, rather than all at once aged 11 and 14. Unions hope this could pave the way for more use of internal teacher assessments.
