The figures, based on answers to parliamentary questions, reveal that in most cases schools that have not been inspected make better progress in improving GCSE examination results than those that have.
Don Foster, the Liberal Democrat's education spokesman who published the analysis, accused Ofsted of failing to carry out its job of school improvement. Critics of the standards watchdog argue that its approach is too threatening and punitive to be effective in improving schools.
But Chris Woodhead, the Chief Inspector of Schools and head of Ofsted, strongly defended its five-year record and said that the difference in the rate of improvement between the inspected and uninspected schools was insignificant.
The analysis looked at the average GCSE score - where an A grade is worth 10 points - for all secondary schools during the first four years of secondary school inspections which began in September 1993. It compared the progress of schools that had been inspected with those that had not.
The average GCSE point score of pupils in about 800 schools first inspected in 1993-4 went up by 2.1 points in the subsequent two years. Over three years it went up by 2.6.
But the pupils in the 900 schools not inspected during those three years showed a bigger improvement - up 2.3 points in 1995 and 2.8 in 1996.
Mr Foster, who analysed the Department for Education data with Charles Bell of the education pressure group Article 26, said: "Ofsted's mission statement is `improvement through inspection'. It is clear from our analysis that, for secondary schools at least, Ofsted is failing in its mission. Once again evidence has been provided that calls into question Ofsted's approach. There is an urgent need for an independent study into the value for money provided by the chief inspector and his team."
The Government's spending watchdog, the National Audit Office, has said that it is considering an investigation into Ofsted.
Mr Bell said: "Our analysis shows that it is possible to identify the effects of Ofsted. And when we do, Ofsted is found to be wanting."
Ofsted said that it had never claimed a link between an inspection and immediate improvement.
Mr Woodhead said: "This sensational and simplistic analysis does not constitute a serious contribution to the debate about inspection. It is statistically suspect and conceptually naive."Reuse content