Part-time higher education should not be seen by the Government as a "substitute" for full-time study, a senior figure warned this week.
The message was delivered as figures came to light suggesting that more than half of part-time students fail to complete their degrees.
Earlier this month, Lord Mandelson justified the Government's decision to cut full-time undergraduate places at a time of record demand by arguing that the three-year honours degree straight after school was not where the Government should focus future growth.
Instead, he said, universities should offer a wider range of courses, including more part-time programmes.
Labour has placed strict limits on the number of full-time undergraduates universities can recruit, but the restrictions on part-time student numbers are less stringent.
But Bahram Bekhradnia, director of the Higher Education Policy Institute, has warned that ministers need to tread carefully.
He drew attention to a study published by the Higher Education Funding Council for England last year, which he said was subsequently "buried". It followed a cohort of more than 16,000 part-time students who entered university in 1996-97, and examined what had happened to them 11 years later.
Leaving aside students of The Open University, of those who declared that their aim was a bachelors degree, 59 per cent left university without a qualification.
"Both the Government and the Conservative Party are promoting part-time higher education, no doubt in part for good reasons, but no doubt in part because part-time students cost the Government less than full-time ones as they do not attract loans for fees and maintenance. They need to be careful," Mr Bekhradnia said.
Part-time study is important because it extends opportunities to people who may not be able to study full-time and, increasingly, to those already in work who need to improve their skills or even change jobs, he said.
The findings of the Hefce study may be an argument for more generous support arrangements for part-timers, he suggested.
"But we should be quite clear that in general, part-time higher education should not be regarded as a substitute for studying full time," he argued.
"The pressures of part-time study are enormous, and this study shows that the chances of success are far lower than for those who study full time.
"We should be under no illusion that it is an easy road to follow and should be doing all we can to encourage young people in particular to study full time."
Claire Callender, professor of higher education policy at Birkbeck, University of London, and an expert on part-time study, said her research showed that about 80 per cent of part-time students work, with many having children.
"Studying as a part-time student is a juggling act, with academic work fitted around other commitments," she said.
In a study she conducted for Universities UK in 2006, 83 per cent of part-timers said they were "too busy" at work, and 77 per cent said they were "too busy" at home, while 71 per cent said courses were more time-consuming than they had expected.