More than 5.4 million employees put in around £640 million worth of unpaid hours every week in 2013, amounting to £33 billion worth of unpaid hours a year, according to research by the Trades Union Congress (TUC).
Its research, which was released to coincide with annual Work Your Proper Hours Day on 28 February 2014, found that the number of people who regularly work unpaid hours increased by 331,000 in 2013, to 5.42 million, the biggest annual rise since comparable records began in 1998.
The research also found that the proportion of people doing unpaid overtime is at its highest-ever level, at 21.2% of the UK workforce, while the average amount of unpaid overtime has also reached a record high of seven hours, 48 minutes a week.
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London is the capital of unpaid overtime, with 900,000 workers regularly putting in more than eight hours of unpaid overtime every week.
Employees in education are the group most likely to do unpaid overtime (37.7%), followed by those in professional, scientific and technical activities (33.3%).
People in their early 40s are the most likely to do unpaid overtime (26.8%), followed by those in their late 40s (25.5%) and late 30s (25.3%).
Frances O’Grady, general secretary at the TUC, said: “Staff across Britain work among the longest hours in Europe, and are not even paid for much of the extra time they put in.
“Staff don’t mind doing a few additional hours during busy periods, but too many employers take this goodwill for granted and forget to thank their staff. Further problems arise when those occasional extra hours become the norm, and staff become overworked and underpaid.
“The many bosses who encourage long hours in the office should re-think their approach as stressed, overworked staff are often unhappy and unproductive.”