Chief inspector Chris Woodhead came out fighting after claims by Liberal Democrat education spokesman Don Foster and independent researcher Charles Bell that GCSE results in inspected secondary schools improved no faster than in non-inspected schools.

Mr Woodhead dismissed analysis of Government statistics gathered in response to Commons questions put by Mr Foster as “sensational and simplistic”.

And he found an unlikely ally in Harvey Goldstein, professor of statistical methods at the Institute of Education, a frequent critic of the service. He said it was “a bad piece of research” and had asked the wrong questions”.

But Professor Goldstein suggested OFSTED had only itself to blame if others chose to conduct their own evaluation of the service.

“OFSTED does not have a good track record. Its own research is of a rather low standard.

“OFSTED should be prepared to put some effort into allowing independent people to come in. It isn’t accountable at the moment,” Professor Goldstein said.

The pound;150 million-pound-a-year inspectorate is already likely to be examined by the National Audit Office.

Charles Bell analysed the GCSE point scores of every maintained non-special secondary school in England during the four years of the just-completed first full cycle of inspections. It compared the progress of those inspected with those that had not.

Comparing the results of the 800 schools inspected in the first year, 199394, it found point scores went up by 2.1 points between 1993 and 1995, while those that had not been inspected improved by 2.6 points.

But OFSTED called the gap statistically insignificant and pointed out the first year had focused on the weakest schools. They were bound to take time to turn around.