Universities are spending millions of pounds on recruiting more poor students, but there is little information available on the best ways of boosting numbers, according to research.
There is a "severe lack of evidence" about what works best in encouraging disadvantaged young people to study for a degree, the Sutton Trust said.
Figures suggest that the numbers of disadvantaged young people going into higher education have risen in the last decade, but the most selective institutions are still dominated by youngsters from richer backgrounds, a brief by the social mobility charity notes.
In total, English universities spent £124 million on "outreach" work this year - schemes designed to encourage poorer youngsters to go to university.
But there is too little robust evidence available to tell them which methods are the most effective, the brief warns.
"There is strong evidence that outreach activities, in general, do succeed in attracting and admitting students from non-privileged backgrounds, but there is not enough evidence indicating which particular initiatives work," it says.
The brief goes on to say that analysis of existing research in the US and the UK suggests that running summer schools, tutoring and mentoring are the methods with the most evidence of success, and that those that work with parents and teachers, and involve youngsters early on to boost their academic achievement, are likely to have a positive effect.
Sir Peter Lampl, chairman of the Sutton Trust, said: " Universities are under considerable pressure to get more disadvantaged young people into higher education but there is a severe lack of evidence about the factors that make some approaches work better than others. With the access gap at our most selective universities still far too wide, we need to get a much clearer picture of what works best."
Universities should spend at least 10% of their "outreach" budgets on evaluating their schemes, the brief suggests.
Nicola Dandridge, chief executive of Universities UK, said: "Universities spend considerable time and resource considering the evidence and evaluating what works best for them in terms of improving participation. This will result in more targeted outreach work with schools and colleges and allocating financial support more effectively.
"Universities work hard with schools and colleges and undertake a whole range of initiatives to raise aspirations and attainment, as well as pressing for better advice and guidance for students. We have made progress but we still have more to do to improve participation rates for disadvantaged students.
"Universities UK is now leading a social mobility advisory group to build on this progress and identify best practice."