Students working during term-time put in an average of nearly 20 hours a week and more than half say it is affecting their studies, according to a survey by the National Union of Students.
More than two-thirds said they were working to meet basic living costs and only 10 per cent were in jobs relevant to their chosen career.
Hotels, pubs and restaurants employ 42 per cent of student workers, and shops and supermarkets a further 31 per cent. While two-thirds are doing the same job as permanently paid staff, 29 per cent are paid less than full-timers.
About 40 per cent have no written contract, although this is now a legal requirement within two months of anyone starting work. Average rates of pay are Pounds 4.37 an hour, Pounds 2 less than the Low Pay Unit's threshold. Hours average 13.3 per week, plus 6.5 in overtime.
Fifteen per cent of students surveyed said their studies have been badly affected by working, with nearly half saying they would have achieved higher grades had they not worked. More than a quarter said it had affected their health. Their main health and safety concern was the danger of travelling alone late at night.
Earlier research by the NUS suggested that two in five students work during term-time.
Andrew Pakes, NUS president, said: "The NUS believes the government must improve the student-support package, with a focus on taking students out of hardship."