Only 7 per cent of families that have a child approaching university age would qualify for the government's new £1,000 student grant, according to official figures.
The Office for National Statistics has confirmed that among families with one or more dependent children aged 16, 17 or 18, just 7 per cent have a total income of £10,000 or less - the threshold below which students will receive the full £1,000 grant.
Experts said the figures showed how inadequate the grant will be for encouraging young people from disadvantaged backgrounds into university.
Claire Callender, the government access adviser credited with persuading ministers to bring back the grant, said: "This shows that not only is the grant itself too low, the threshold for receiving it is set too low. It means that only a very few school-leavers will actually be eligible for the full grant, so a key targeted group is not being targeted sufficiently."
Professor Callender, professor of social policy at South Bank University, said that mature students over the age of 25 would be the main recipients of the grant.
A spokeswoman for the Department for Education and Skills said it estimated that 30 per cent of all students would be eligible for the full grant, which will be introduced in 2004 and reviewed in 2006. She said that 15 per cent of those receiving the full grant would be mature students.
A further 13 per cent of students, with family incomes of between £10,000 and £20,000, would be eligible for a partial grant below the maximum £1,000. The DFES said it had not yet decided what the sliding scale or minimum payment would be.
Professor Callender said the fact that just 43 per cent of students would receive at least some grant under the new system was "very disappointing", compared with the 70 per cent who got much more generous grants under the old regime, when higher education was free.
Next week she will publish a report for Universities UK on student attitudes to debt that will cast doubt on the government's claim that its new student-support package would encourage students to go to university.
The report will show that at least one in four disadvantaged students with the right qualifications doubts that they would go to university because of associated debt.
"Grants can play an important role in helping to reduce debt... but it is far from clear that the new grant will be adequate enough to help them overcome that debt aversion," Professor Callender said.
Lecturers' union Natfhe has said the grant is "miserly" and available only to families on "ludicrously low" incomes.
Mandy Telford, president of the National Union of Students, said that the NUS "must employ every campaigning tool" to fight against increasing student hardship and debt. The union is hoping to organise a one-day shut-down in universities with trade unions.
Criticism of the grant level this week led education secretary Charles Clarke to suggest that when grants are reviewed in 2006, they are likely to be increased.
Professor Callender's comments follow the Unite student living survey that showed that 38 per cent of students say that they are "seriously worried" about the debts brought by a university education. The figure rises to almost half of students from working-class backgrounds. Those surveyed said they owe £4,602 on average, an increase of £399 from last year, and they anticipate owing £8,816 at the end of their studies.