Teenagers from working-class backgrounds are up to three times less likely to attend a top university than their financially well-off classmates, despite sometimes having the grades needed to gain a place, research suggests.
A new study has suggested that the disparity in the numbers of advantaged and disadvantaged youngsters attending university is not solely related to their academic performance at school.
The study, commissioned by the Sutton Trust, looked at the numbers of children attending top universities in England, the US and Australia who came were from different economical backgrounds.
It found that in England, children with professional parents are about three times more likely to attend a Russell Group institution - considered among the best in the country - than those from working-class homes.
About 73 per vent of this gap was attributed to the pupils' previous academic achievement, the study concluded. It argues that some of the discrepancy is "unexplained" and it could be that many students from lower-income families with decent grades may be choosing to go to other universities.
The research also found that the real cost of going to a top university in the US may be less than attending a similar institution in England, despite higher headline fees.
It concluded: "Although academic achievement up to age 18 can explain a great deal of the socio-economic gap in elite university access, it does not completely remove it. At least a quarter of the difference in England, the US and Australia is not explained by academic ability."
The study notes that at the top English universities only one in eight young undergraduates come from "lower" occupational backgrounds, compared to more than half at some of the newer institutions.
More than two fifths of students attending Oxford and Cambridge went to private school while at some of the modern universities the figure is two or three per cent, the report said.
Dr John Jerrim, of the Institution of Education, University of London, who conducted the study, said: "Although academic achievement is an important factor, a substantial proportion of the elite university access gap in each country remains unexplained.
"This suggests that there are working-class children who, even though they have the grades to attend, choose to enter a non-selective institution instead."
The study, due to be presented at a Sutton Trust summit on university access, also examined the cost of a university education.
It found that while the "sticker price" or headline fees at top private US universities are usually much higher than those in England, where the maximum fee allowed is £9,000, generous financial support means that poorer students often graduate with less, or no, debt.
Average fees are £9,000 a year at Oxford and £24,200 at Harvard, the study says.
Once living costs and accommodation is taken into account, the overall price of going to Oxford is around £16,600 a year while Harvard is £37,333, it argues.
But a Harvard student from a family with a household income of £27,500 would only be expected to find £2,019, while a similar student at Oxford - which does offer generous financial support - would need to find around £11,300.
An Oxford student with a household income of around £10,000 would have to contribute around £3,550 and a Harvard student would pay around £865.
In England, the money would be repaid with interest once the student was working, with payments linked to salary, and in the US it is usually paid off through a work-study programme.
Sutton Trust chairman Sir Peter Lampl said: "This new research confirms that there are many able children either not applying or not being admitted to the best universities, and this is true internationally."
Dr Wendy Piatt, director general of the Russell Group, said "students not only need good grades, they need them in the right subjects".
An Oxford University spokesman said the research failed to acknowledge that Oxford students could earn money by working during their vacations or to make any distinction between US system where fees have to be paid up-front, and the UK's loan-based system where repayments are made after graduation according to income.
He added: "Simply, like is not being compared with like."
Additional reporting by Press Association