Policies to attract more working- class students into higher education may be based on "flawed" research with incomplete data or suspect methodology, funding chiefs have been told.
The interim report of a review of the evidence about widening participation warns that conclusions drawn about the impact of new admissions policies or outreach work may not be reliable.
The report was commissioned by the Higher Education Funding Council for England. It will use the conclusions of the final report later this year to target funding for projects most likely to succeed in attracting underrepresented groups into higher education.
After logging 1,200 research reports, the review team has told Hefce:
"There are three general flaws that characterise research concerning widening participation in education. These are the lack of controlled interventions, the lack of appropriate comparator groups... and the lack of proportionate analyses of the figures involved.
"So common are these flaws that they are not generally remarked on, not picked up in peer review and not taken into account when attempting to draw warranted conclusions from one or more studies."
The review - carried out by academics at York and Staffordshire universities and the Higher Education Academy - then turns to the claim made by the Sutton Trust that its outreach work and summer schools help to widen participation.
The report states: "This claim manages to combine ignorance of the need for a comparison or control group with the flaw of combined and uncontrolled multiple interventions.
"The Sutton Trust can have no idea whether any of these several interventions was effective... or whether the same changes in university applications would have occurred in the absence of them."
The trust told The Times Higher that it would be "happy to consider its recommendations" when it had been able to study the detail of the report.
The review team will now examine evidence about the effect of government policies such as SureStart and literacy and numeracy initiatives in schools as well as measures to widen access to higher education.
One concern is that "no single large-scale dataset exists to establish that this unfair underrepresentation is in fact so".
Stephen Gorard of York University, who is leading the review, said:
"Perhaps the most surprising finding is how little we know about patterns of participation in higher education and how to change them."
Hefce said that the purpose of the review was to ensure that policies were supported by "robust evidence".
"We have no wish to duplicate existing work, but we do need to understand where there are gaps or shortcomings in the existing evidence," a representative said.