Undergraduates who work part-time during term-time are likely to do less well academically than their contemporaries, research suggests.
Working in term-time can affect results on graduation day, says research
When working 15 hours a week, the odds of working students getting a first class or upper second degree were found to be 62% of similar non-working peers.
The survey of 1,500 UK-based students also found working students tended to come from disadvantaged backgrounds.
The study is likely to fuel debate about the future of student funding.
The research, commissioned by the Higher Education Funding Council for England (Hefce) and Universities UK, questioned 1,500 final-year students from seven universities across the UK.
Almost 70% of those who took part said they were struggling financially and three quarters were concerned about paying back their debts.
Half of those surveyed were combining paid work with their studies and half were focussing solely on their academic work.
Comparisons of students considered to be of a similar ability found those in employment tended to be awarded lower marks in assignments and were less likely to gain the higher classes of degree.
Researchers also noted that those students working during term-time were more likely to include women, people from ethnic minority groups and people from poor backgrounds.
Working students themselves felt their studies suffered as a result of their employment.
Examples given included producing poor assignments, missing lectures and having difficulty accessing libraries and computer facilities.
More than 80% said that they spent less time studying because of their term-time jobs and nearly three-quarters spent less time preparing their assignments.
There are also concerns that working students miss out socially
Professor Claire Callender from London South Bank University, one of the authors of the report, said there was also evidence that working students were losing out on social and leisure activities.
"Some students' university experience is of a very different calibre to others," Professor Callender told the BBC News website.
"The implications are far-reaching. Overall, the vast majority worked because of financial need.
"The longer hours they worked, the more detrimental the impact on their studies."
Professor Callender said attempts to widen participation in higher education focussed on getting people into university and did not examine what happened once they embarked on their university career.
"The key concern is that this is an issue that is faced up to and dealt with," she said.
"The sector can't ignore it. Term-time working is going to be a permanent feature on the higher education landscape."
The research findings have already proved controversial.
Last week , the Times Higher Educational Supplement reported that Universities UK had cut "politically highly contentious" parts of the study.
But a spokesman for Universities UK said the results of the research were known in December 2002 so it was not realistic to talk of "suppressing" the work.
'Hard to spin'
The National Union of Students said the research findings were worrying.
"The statistics found in this report are hard to spin," said NUS national president Kat Fletcher.
"Students working during term-time and putting their degree in jeopardy as a result. Income from part-time work going towards 'things needed to survive' such as food and rent."
New finance package
Higher education minister Bill Rammell said the study did not reflect the impact of the new student financial package where students will not have to pay until they have finished studying and are earning over £15,000.
"With the reintroduction of grants worth £2,700 a year and bursaries offered by many universities, students should need to find less cash to support themselves while they are studying," he said.
"Graduates continue to earn a substantial return from their degree and we should celebrate this fact."
But shadow skills minister Stephen O'Brien said: "The damaging effect of students working more hours at university was just one of the many concerns raised before Labour introduced tuition fees.
"It is a serious issue which the government must not duck, despite their usual disclaimers of responsibility."
And Liberal Democrat education spokesman Edward Davey said: "This report is bad news for the government's tuition fees policy."