The class divide among university students widened during the second half of the 1990s, a study says.
The gap between the proportions of middle-class and working-class children almost doubled between 1994-5 and 2001-2, researchers say.
Tuition fees, introduced in 1998 in England and Wales, had failed to deter middle-class young people entering higher education, says the study by London University's Institute of Education and the London School of Economics.
And GCSE exams had helped "less able but wealthier students" to stay on, they said.
From 1994-5 to 2001-2 the proportion of those from families in areas with average annual incomes of below £10,950 rose from 0.89% to 1.77%.
But the rate in areas with an average income of £21,890 increased from 10.04% to 19.16%.
The researchers also said the GCSE exam, introduced for 16 year olds in the late 1980s, had been more "accessible" than the old O-level and CSE.
It had created a "massive jump" in those gaining qualifications and had "removed a potential ability hurdle for students".
This, the authors argued, might "explain why the subsequent expansion of higher education seems to have enabled wealthier children to increase their chances of going to university to a much greater extent than poorer children".
However, the report says: "Children from all socio-economic backgrounds are considerably more likely to go to university in 2001, as compared to 1994.
"In fact, the growth in higher education participation amongst poorer students has been remarkably high, mainly because they were starting from such a low base."
The authors, Anna Vignoles, Fernando Galindo-Rueda and Oscar Marcenaro Gutierrez, said "income-driven inequality" was part of a long-term trend, related to the expansion of universities.
The social make-up of students also changed during the period studied.
Of those participating in higher education in 1996, around one third were from a professional, managerial or technical background. By 2000, this had risen to 42%.
Meanwhile, 8% of those participating in 1996 had a father with a degree, rising to 14% by 2000.
A spokesman for the Department of Education and Skills said: "Aspirations towards higher education are often formed before the completion of compulsory education.
"That is why we have brought together existing widening participation initiatives to form a unified Aimhigher Programme which, based on collaboration among schools, colleges, universities and others, will seek to raise aspiration, attainment and application levels amongst groups currently under-represented in HE."