Tony Blair's drive for education reform was dealt a blow today as research shows there was no convincing evidence that specialist status itself is improving secondary schools.
The Prime Minister has used the apparent success of specialist schools to justify his plans for more change.
But a report, published today by the Research and Information on State Education trust, states that "much of the evidence provided by the Government has been inconclusive or methodologically suspect".
"It is clear that the majority of specialist schools are highly effective - four-fifths were judged to be so in a survey by Ofsted," the study says.
"But whether this is due to their selection practices (overt and covert) or to their being already highly effective in order to obtain specialist status is not clear.
"There is no proven causal link between the improved performance of these schools and their specialist status."
Mr Blair praised specialist schools in his introduction to the education white paper.
In it he says: "We have seen how specialist schools - with external sponsors, strong leadership and a clear sense of mission, driven by their acquisition and retention of specialist status - have improved faster than other comprehensives."
Researchers Frances Castle and Jennifer Evans of London university's institute of education acknowledge that the three-quarters of secondary schools which now have specialist status are getting better GCSE results than the remainder.
They also say that pupils in specialist schools get better GCSE grades than similar students in other schools.
But their report notes that schools can only get specialist status if at least 25 per cent of their pupils are already gaining five A* to C grades.
It said that research was needed to find out if specialist schools were cost-effective, as a typical school in the programme can receive around Pounds 600,000 extra over a four-year period.
It also said that too little was known about the impact that specialists have on neighbouring schools, as there were "indications of increased social polarisation in some areas".
The Specialist Schools and Academies Trust (SSAT) has defended the schools by publishing a series of evaluations on their value-added results produced by Professor David Jesson, from York university.
But the new report said the evaluations did not take enough factors into account and that it was wrong to draw evidence from them suggesting a causal link between specialist status and results.
The SSAT said it was unable to comment on the report.
Specialist schools - what do we know? will be published on www.risetrust.org.uk