Ron Glatter, a visiting professor at the Open university and Warwick university, said there seemed little to back up several of the inspectorate's findings, including an assertion that specialist schools did not disadvantage neighbouring non-specialists. Professor Glatter has suggested that Ofsted's eagerness to praise the schools raises questions about its independence, as the scheme to expand specialist schools has strong support from ministers.
Ofsted published a broadly positive report on specialist schools in February, which said the status was a catalyst for improvement.
In a statement promoting the study, the inspectorate said that it had carried out a survey which "found no evidence that other schools located nearby were disadvantaged by having a specialist school in the area".
Professor Glatter said he had been surprised to find no details of the survey in the published study.
He contacted the watchdog and obtained a short report giving details of the survey of 60 specialist schools and its findings.
It said that there was "not sufficient evidence" to indicate whether or not the schools and their neighbours were improving at different rates at GCSE level.
Professor Glatter said: "That has a very different implication to the statement in the release. In fact, there is evidence in the main report that non-specialists which are neighbours of specialist schools have a lower level of attainment than non-specialists do nationally."
Professor Glatter said that he had been struck by the "purple prose" written by David Bell, chief inspector, about specialist schools, which he said would not have been out of place in a minister's speech.
"There still seems a big question about how independent his organisation really is," Professor Glatter said.
An Ofsted spokeswoman stressed that it was an independent organisation.
Although its survey had been inconclusive about rates of improvement at GCSE, it had found that non-specialists which were close to specialists had similar levels of attainment to others.
The Ofsted report also highlighted that more than half of specialists had failed to meet GCSE targets in their chosen subjects.