Chief inspector Chris Woodhead has launched an outspoken attack on the Government's regime of tests.
Mr Woodhead says the tests are so unreliable that schools, parents and politicians have been left in the dark about pupils' progress.
His views, outlined in a lecture this week at the London School of Economics, will be an embarrassment to the Government, which has staked its reputation on improving test results by the end of its term in office.
According to the chief inspector, the national curriculum tests, given to seven, 11 and 14-year-olds, are vague and unreliable. He accused schools of administering them "creatively".
This unprecedented condemnation of the work of a rival quango has angered officials at the Qualifications and Curriculum Authority, which runs the tests, and at the Department for Education and Employment.
The QCA has already come under fire after problems collecting last summer's test results. This hitch cost the quango an estimated #163;2 million and delayed the publication of primary school performance tables.
Ministers had hoped parents would see the results this autumn, but the tables will now not be available until February.
However, Mr Woodhead said the unreliability of current tests made it impossible to measure improvement in pupils' performance. "There are no reliable national curriculum tests," he said. "I have not got a lot of faith in it for three reasons.
"First, I am not sure the tests are the right tests. The concept of levels is a very vague one; we need standardised tests in literacy and numeracy.
"Second, the tests have changed quite significantly over recent years so it is impossible to compare like with like.
"Third, a lot of individual tests are being administered in a creative way by some schools."
However, the Government sets great store by the tests and insists that 80 per cent of 11-year-olds must achieve the required standard in the English tests, and 75 per cent in maths by 2002. Only 65 and 59 per cent respectively are currently performing at that level and Education Secretary David Blunkett has said he will resign if the targets are missed.
A DFEE spokesman said: "The Government believes the current tests offer a reasonable standard on which to judge achievement. There are no plans to alter them."
A spokesman for the QCA said: "Successive governments have used the national tests as the most reliable way of measuring children's progress in the core subjects of the national curriculum and against national targets.
"The tests cannot be bettered as a way of giving teachers and parents information on children's progress at the end of each key stage."
An OFSTED spokeswoman said: "These are all concerns that have been expressed before. It is right that the chief inspector should raise issues about education policy in order to get people thinking about them.
"He was primarily putting the views of other people, although he certainly shares some of them. The system is not perfect yet. But this is what we have at the moment."
This is not the first time Mr Woodhead has spoken against the national tests.
Last year a school he had singled out for special praise, the Strand junior in Grimsby, performed badly in the tests, ranking 50th from bottom in national performance tables of around 14,000 schools.
Then the chief inspector commented: "Either OFSTED is rubbish or performance tables are rubbish."