Britons worked nearly two billion hours in unpaid overtime in 2011, according to the Trades Union Congress (TUC), which said the total is the equivalent of one million full-time jobs.
"While many of the extra unpaid hours worked could easily be reduced by changing work practices and ending the UK’s culture of pointless presenteeism, a small number of employers are exploiting staff by regularly forcing them to do excessive amounts of extra work for no extra pay," Brendan Barber, the TUC's general secretary, said.
“This attitude is not only bad for workers’ health, it’s bad for the economy too as it reduces productivity and holds back job creation."
Around 5.3 million workers put in an average of 7.2 hours each of unpaid overtime per week, the TUC said - equivalent to more than £5,300 per year in lost wages.
Staff in London and the South East are the most likely to put in extra hours, with a quarter of workers in both regions working overtime without reward.
Daniel Naftalin, an employment expert at law firm Mishcon de Reya, warned that, while there was an issue with "unreasonable" overtime, the response has to be proportionate and "nuanced".
“It is important to differentiate between those who are choosing to work longer hours in order to further their career or because they are conscientious, for example, and those who are being unreasonably forced to," he said.
"The UK workplace has moved on from the days of master and servant and, if an employee is in the second category, they should seek legal advice and utilise the employment rights available to them such as the working time regulations, harassment laws and contractual rights."
Forcing employees to work to rule would risk the UK's place as an attractive destination to do business, Naftalin said.