Almost a quarter of UK workers admit to working 10 or more hours of unpaid overtime per week.
The finding, from a poll by recruitment company Select Appointments, adds further weight to evidence that UK workers are saving their bosses billions of pounds by working for free.
A study by the TUC in January estimated that unpaid overtime totalled £23 billion last year, with workers losing some £4,650 as a result each.
The latest poll, of more than 1,000 people, found a further 20 per cent admitted to working more than five hours extra a week.
Select Appointments HR director Bill Dykes said: "Britons seem to be addicted to their jobs and work the longest hours in Europe.
"Although many workers crave work-life balance, work seems to take over their very existence in a bid to climb the career ladder.
"It is difficult to assess whether businesses are responsible for piling the work onto their employees, or the employees themselves feel they have to put in the hours to keep their jobs," he added.
The only "encouraging" sign, he argued, was the finding that more than half of those polled – 51 per cent – worked fewer than five hours of unpaid overtime a week.
The TUC's study had calculated that if workers had done all their unpaid overtime at the beginning of the year, they would have worked for free right up to Friday 25 February.
