Brendan Barber, General Secretary of the TUC, said: “The recession has forced many employees to do fewer hours in an effort to save jobs and this has also had an effect on the amount of unpaid overtime worked.

“This flexibility and the sacrifices made by staff has saved jobs and kept companies afloat.”

On the positive side, there has been a small decrease in employees who regularly work unpaid overtime. Since 2008, when the TUC last analysed official statistics, 168,000 fewer people have said that they regularly work extra hours for no pay.

The flexibility of the UK’s labour force is credited for the lower than expected rise in unemployment in recent months. Some employers have spoken of a “spirit of collaboration” that has seen staff make sacrifices to save jobs.

Mr Barber said: “Most employers are understandably focused on fighting their way through the recession. But they shouldn’t forget that working cultures such as pointless presenteeism, which keeps people at their desks for no good reason, is not just bad for staff, but bad for business too.

“Millions of people are still working far too many hours and often they are not even being paid for it. This long-hours culture causes stress and damages people’s health.”

Of the five million employees who worked unpaid overtime, nearly 900,000 regularly worked more than ten hours a week unpaid.

Workers in Northern Ireland, the East Midlands and London were the most likely to do more than 10 hours of unpaid overtime a week. London, where a long-hours culture exists, has the largest number of workers — some 166,422 — doing more than 10 hours of unpaid overtime a week.

A CBI spokesman said: “Working long hours is not for everyone, but, as the TUC recognises, in times of recession staff will sometimes have to go the extra mile. CBI research also shows that nearly half of employers are offering staff more flexible ways of working during the recession, which helps businesses get through tough times and gives staff a better work-life balance.”