Ofsted is carrying out sub-standard inspections which fail to properly investigate schools' performance, an independent think tank has said.
A report by Civitas casts doubt over the reliability of Ofsted's verdicts on schools.
It said inspectors relied too heavily on test and exam performance data to make judgements, and claimed that in some cases, inspectors were using out of date information.
The report, Inspecting the Inspectorate, came just days before Ofsted publishes its annual report into schools.
In one chapter, Ofsted inspector Sarah Drake wrote: "Validated exam and test data are central to the inspection team's information gathering. Delays in getting validated statistical data about Sats, GCSE and other test results mean that what we are using can be up to 16 to 18 months out of date."
The report suggested the introduction of "short, sharp" inspections in 2005 had meant the number of inspectors and the time spent in schools has been cut.
It said this meant inspectors could not carry out a thorough investigation.
It claimed Ofsted's plans to cut their budget did not provide value for money, for example the cuts meant specialist inspectors no longer carried out inspection on specific age groups.
And as performance data only tells inspectors about the subjects pupils have been tested in - for example, in primary schools children are only tested in English, maths, and science - inspection of other subjects could go unchecked, the report said.
The authors added that non-academic teaching was also overlooked.
