The key to widening access to university is to persuade more teenagers to stay at school after 16, university leaders say.
Ivor Crewe: Tackle school drop-out rate
Figures this week showed no increase in the proportion of poorer students in England going to university, despite a general upward trend.
President of Universities UK, Professor Ivor Crewe, says nine out of 10 pupils studying at 17 go on to university.
England's drop-out rate is one of the highest in the developed world.
"The single most critical factor in the inequality [about who goes to university] is the high drop-out rate from education at age 16," Professor Crewe told a conference on university access.
"Improvement of staying-on rates post-16 is the key to widening access to those from disadvantaged groups. To obtain two A-levels is almost a passport to higher education."
He said the proportion of 17-year-olds in the UK in education or training was just 76% - the fifth lowest among 28 OECD countries.
A report from the Higher Education Funding Council for England published on Wednesday found wide disparities between different areas in terms of university entrants.
It said teenagers in the richest areas could expect a better than 50% chance of going to university, while in the poorest neighbourhoods it was 10%.
Universities have been under pressure to increase the number of students from poorer backgrounds.
The government has set up the Office for Fair Access (Offa), a watchdog which aims to ensure people from poor backgrounds have a fair chance of going into higher education.
Universities wishing to increase their tuition fees up to a maximum of £3,000 a year from next year have to show Offa how they aim to widen the pool of applicants through offers of bursaries and other outreach work.
Professor Crewe told the conference, in London, that most universities were likely to earmark at least 20% of their additional income for bursaries.
He added that proposals from Mike Tomlinson to shake up the education system for 14 to 19-year-olds would also help to widen access.
He said the plans would lead to "clearer progression routes to higher education, in particular from the vocational side".