Chief inspector David Bell has sent new guidance to his staff after admitting that changes at the Office for Standards in Education could be responsible for the sharp rise in the number of failing schools.
Inspectors may have misinterpreted the inspection framework introduced last September and focused too much on schools' weaknesses, he said.
Concern about the number of complaints from schools led Mr Bell to send the guidance to all inspectors in England.
Ofsted is also planning a sharp reduction in the number of inspections during the next school year in the run-up to the introduction of another framework in September 2005.
"Too many letters suggest to me that inspectors and inspection teams are not always getting the balance of the judgments quite right," said Mr Bell.
The guidance says that inspectors need to ensure that they are using a balanced evidence base and should not rely too much on the grades given to observed lessons. It also stresses that inspectors should "celebrate a school's main strengths as well as diagnose its weaknesses".
Mr Bell's latest guidance follows Ofsted's annual report, which revealed that the number of inspection reports failing to meet the expected standard rose by more than 50 per cent during the past two years.
In 2003-4, 93.1 per cent of reports were satisfactory compared with 95.6 per cent in 2001-2 and a target of 95 per cent.
There has been a 35 per cent increase in the number of schools judged to be failing since the new framework was introduced.
Mr Bell attracted controversy when he warned schools that satisfactory is no longer good enough to raise standards. Schools are now judged to be unsatisfactory overall if 10 per cent of teaching is unsatisfactory. Until last September, that figure was 20 per cent.
Some educationists argue that this has led to good schools being unfairly criticised. In one case, Banham primary in Norfolk received an apology from Mr Bell after inspectors wrongly identified serious weaknesses at the school.
In evidence to Ofsted, the Secondary Heads Association said: "At a time when the chief inspector is reporting improvements in school leadership and teaching quality, it is extremely disturbing to find that more schools are being placed in special measures or serious weaknesses. This can only have resulted from the new inspection framework."
It also states that schools which were judged to be "good" in their last inspection and have since improved are now only rated as "satisfactory".
In February, Mr Bell announced plans for a further overhaul of the regime that would introduce on-the-spot inspections, put more emphasis on schools'
self-evaluation and cut by half the the number of days inspectors spend in schools.
The proposals have received a mixed response from teachers' leaders. John Bangs, head of education at the National Union of Teachers, said the proposals could exacerbate certain problems with the current system.
He said: "We are seeing irrational and punitive behaviour by inspectors.
Recent cases show inspectors taking a hard line with complaints from schools in an effort to keep them from Ofsted. Too many inspectors are desperate to preserve their contracts."
Ofsted overhaul, 8-9
CHANGE OF TACK: THE NEW GUIDANCE
Inspections should confirm and celebrate a school's main strengths as well as diagnose its weaknesses
* The time taken to diagnose a school's weaknesses should not be a factor in making judgments about a school
* Inspectors should not allow the pursuit of a particular evidence trail to skew their findings
* Evidence should be drawn from the full range of sources
* Over-reliance on lesson observations should be avoided
* Judgments of the whole school should take into account the overall characteristics of a school and not rely solely on data
* Inspectors should use more than just a checklist of a school's weaknesses when they are deciding whether to designate it as one which is underachieving, needs to be placed in special measures or has serious weaknesses.