Ofsted is carrying out sub-standard inspections which fail to properly investigate schools' performance, an independent think tank said today.
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too heavily on test and exam performance data to make judgments, and claimed that in some cases, inspectors were using out of date information.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statistical data about Sats, GCSE and other test results mean that what we are using can be up to 16 to 18 months out of date."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.
Tim Benson, head of a primary school in east London said "During our inspection I forced our team to attend my school orchestra rehearsal; 60 children all playing orchestral instruments. The children told of how they were to sing - we have a splendid choir too at Nelson Primary School - at the Festival Hall and the Excel Centre. Other children reported coming top of Newham's school football league. Not one word on sport or music was included in our final inspection report."
He added: "Having lived through Ofsted, in all its transformations, it is failing, in my view, in that it still appears to be destructive where it should be constructive.
"It currently helps to create a culture of fear in our most vulnerable schools: the very schools that need the most encouragement and support."
There is also a feeling that inspectors make up their minds about schools before they have even carried out their inspection.
In a foreword to the report, Barry Sheerman, chairman of the schools select committee, said: "There is overwhelming evidence to suggest that the present inspection regime is consistent with our reliance on the quantifiable.
"Recently Ofsted's inspection process has become shorter and is euphemistically referred to as 'the lighter touch'.
"Many people believe that more attention should be paid to inspection reports than to the results of the key stage tests or GCSE and A level results, yet, as inspection reports and results become increasingly interchangeable, many educational practitioners feel that the inspectors' minds are made up long before they observe the quality of teaching or the atmosphere within a school."
Report editor Anastasia de Waal said: "Parents have been known to move house on the basis of what Ofsted says, but the bottom line is that Ofsted reports are not reliable guides to school quality."
Ofsted denied the assertion that its inspectors judged schools on the basis of exam results, and said the suggestion showed a lack of understanding about how inspections work.
A spokeswoman said: "Ofsted considers the progress learners make in the school, the background of its pupils, the school's own self-evaluation and evidence from the direct observation of lessons.
"All this evidence enables school inspectors to make more than 30 judgements including safeguarding, equalities, behaviour, teaching and the effectiveness of leadership and management.
"Only one judgment relates directly to exam or test results.
"Ofsted inspects schools to inform parents about standards and to help improve the services they offer children and young people."
Ofsted said a survey by Ipsos-Mori had found that parents support inspection, while post-inspection surveys show headteachers agreed that inspections identified the right issues for improvement.Reuse content