A record £640million-worth of unpaid overtime was worked last year following a huge increase in the number of employees putting in extra hours for free, a new study has revealed.

The TUC said 5.4million people worked extra hours for no pay in 2013, an increase of 331,000, the biggest annual rise since records began in 1998.

Over a fifth of the UK workforce now works unpaid overtime, averaging almost eight hours a week per employee, said the union organisation.

Officials suggested that workers were coming under increasing pressure to do extra hours without being paid.

The problem was said to be worse in London, with an estimated 900,000 workers in the capital regularly putting in over eight hours of unpaid overtime every week.

Workers in their early 40s are most likely to do unpaid overtime, especially in education or in professional, scientific and technical jobs.

TUC general secretary Frances O’Grady said: “The British workforce is often unfairly portrayed as a nation of skivers and shirkers, but the reality is exactly the opposite.

“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 over-worked and under-paid.

“The many bosses who encourage long hours in the office should rethink their approach as stressed, over-worked staff are often unhappy and unproductive.

“If there really is much too much work to go round, employers might want to consider taking on new staff. There are 2.3 million unemployed people across the UK who would be glad of the chance.”

The study was published to mark Work Your Proper Hours Day, the first day of the year that those doing unpaid hours effectively start to get paid for the work they do over the course of the year, according to the TUC.