John Bangs, head of education at the National Union of Teachers, says an over-emphasis on preparing pupils for tests is limiting the amount of time available to teach other subjects.

Parents have become so concerned about their children passing tests that they have been adding to the pressure on teachers to devote more time to revising for the tests.

John Bangs warns that too great an emphasis on tests takes away time for other parts of the curriculum

"There is a tremendous pressure on schools to achieve good results in tests - from government, local authorities and parents. The impression has been given to parents that these tests represent enormously high stakes."

As a consequence, Mr Bangs says schools are pushed into spending less time on non-test subjects, such as arts and humanities, and more time practising for the tests.

Although the tests for seven, 11 and 14-year-olds are not public exams with passes and failures, parents have become increasingly keen to see their children perform well in the tests.

The publication of primary school league tables based on the tests and the government's setting of targets to improve the national average for test results has raised their profile and parental anxiety.

Publishers have reported a trebling in sales of revision guides and past papers for national tests, as parents worry that their children will not perform as well as possible.

When schools are drilling their pupils to pass particular test papers, Mr Bangs says the usefulness of tests as a guide to ability is limited.