Tuition fee costs at US universities will rise once again at double the rate of inflation this year, nearing $40,000 (?21,000) at some elite schools, including Harvard University.

The tuition-fee announcements, which come each spring, coincide with a report that says that low and middle-income students are increasingly being excluded from US higher education.

The report, by the non-profit Century Fund, shows that three-quarters of students at the nation's top 146 universities come from the wealthiest socioeconomic quartile. Some 3 per cent come from the poorest quartile.

A student at a top school, the report says, is 25 times more likely to be from a rich family than a poor one.

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"As presidential candidates debate how to improve equity in higher education, attention should be focused on America's low-income students," it says.

Even as the report was being printed, Harvard announced that its tuition fees would increase by more than 5 per cent - double the rate of inflation for the second year in a row - to a little under $40,000, including room and board. A university official called it "a very reasonable increase".

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The cost of a US university education has outpaced family income for 25 years. Much of the increase has been fuelled by growth in the costs at private universities. Now, with government subsidies being cut, public universities are also announcing double-digit price hikes.

Last year, tuition fees went up 14 per cent at public four-year universities to an average that was equal to one-sixth of the total pre-tax income of the average middle-class family. And this year may be worse.

For example, Texas no longer provides government subsidies to universities based on enrolment, resulting in huge cuts that have triggered a 26 per cent increase in tuition fees at Texas University and a 24 per cent increase at North Texas University.

Since 1981, the cost of a four-year public higher education has increased by 202 per cent, while the consumer price index rose 80 per cent.

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Other studies estimate that the cost of tuition fees prevents half of university-eligible high-school graduates from attending a four-year university and one out of five from receiving any higher education at all.

Meanwhile, financial aid that is meant to help the neediest students is being redirected to recruit those who stand out academically in the increasingly competitive university admissions process.

"Colleges are beginning to act less like non-profit educational institutions and more like market players, using financial aid as a way of attracting talented students away from competitors, rather than as a method of helping those who need it most," the Century Fund report says.