Working-class teenagers are around three times less likely to go to a top university than their richer classmates, even though they may have the grades to win a place, research suggests.
A new study concludes that the difference in the numbers of advantaged and disadvantaged youngsters going to university is not just down to their academic achievement at school.
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.
The study, commissioned by the Sutton Trust, looked at the numbers of children from different backgrounds going to top universities in England, the US and Australia.
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.
But this means that more than a quarter of the difference cannot be explained in this way.
"This suggests that there are significant numbers of working-class children who, even though they have the academic ability to attend, choose to enter a non-selective institution instead," the research found.
It also says: "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.
It goes on to say that in England and the US children from disadvantaged homes account for around one in 20 of the students enrolled into the most selective universities - those that tend to ask for the highest grades.
At the same time, youngsters from richer backgrounds account for more than half of enrolments.
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 looked at 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 not be paid upfront but 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".
"This is especially important because entry into some courses, like Medicine or English is very competitive. It is also the case that some very bright students are not encouraged to apply for leading universities. We cannot offer places to those who do not apply or who have not done the right subjects to study their chosen course. "
An Oxford University spokesman said t he 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."
A Business Department spokeswoman said: "We want everyone with the desire and talent to be able to study at university, irrespective of their background. Last year the proportion of disadvantaged English 18-year-olds applying to university was at its highest level."