Speaking at the London School of Economics, Mr Woodhead told researchers that he did "not have a lot of faith" in the tests taken by seven, eleven and fourteen year olds.

The tests, according to Mr Woodhead, were not "the right tests" and were being "creatively" administered in schools - a comment reflecting concern over the fairness of schools' implementation of tests.

Test results are a key measure of the government's educational reforms

Mr Woodhead's remarks - described by headteachers' union leader David Hart as a "bombshell" - will cause particular embarrassment, as the government is making an improvement in test results as a centrepiece of its campaign to raise standards in schools.

The government on Thursday is set to commit itself to meeting targets on tests, including requiring 80% of 11-year-old pupils to achieve expected levels of literacy by 2002, compared to the current 63%.

Earlier this year, the government faced criticism when test results for maths showed a decline, leading to calls for renewed efforts from the Department for Education.

But if these tests are unreliable measures of ability, then the detailed figures used by the government to set targets are rendered meaningless.

David Hart, General Secretary of the National Association of Head Teachers, said that for the chief inspector "to cast doubt on the validity of tests, and to imply strongly that schools are cheating" could "undermine a central plank of the government's standards agenda".

Mr Woodhead expressed concerns about the worth of these tests, saying that they needed to be more "standardised", that year-on-year comparisons were unreliable and that there were problems with how tests were administered by teachers in the classroom.

A spokesman for the Office for Standards in Education said that these remarks were not intended as any kind of policy statement, but were thoughts on how testing might be improved.

The remarks, made at a small seminar, were described by Ofsted as reflecting the questions asked by teachers that Mr Woodhead met on his visits to schools.

Fears over the fairness of the tests and rumours of teachers' helping pupils to pass had to be addressed and doubts had to be eradicated, the spokesman said.

Mr Woodhead's comments were intended, the spokesmen said, to contribute to improving the testing system, rather than attacking it.