A quarter of a million more people aged in their late 50s and early 60s are now working unpaid overtime than were a decade ago, a report says.
The TUC said the rise was out of line with much of the working population where unpaid overtime had fallen.
It said this reflected people working past normal retirement amid concerns about income in later life.
The trade union organisation said it had found that 660,000 workers in this age group put in unpaid hours in 2011.
That was 250,000 more than had done so in 2001.
The TUC found that five million people regularly worked over their hours for no extra pay.
It said this was down to the UK's long-hours culture, which it said was particularly prevalent among those in their 30s and 40s in managerial roles.
But looking over the past 10 years, the TUC said the biggest rise in those undertaking free work was among those in their 50s and 60s.
The union body said the long-hours culture could cause health problems, stress and low morale and is calling for employers to encourage staff to leave on time.
TUC general secretary Brendan Barber said: "Over the last decade, more people are working well into their 60s - and many of them are putting in extra unpaid hours too.
"A lot of older workers are keen to reduce their hours as they approach retirement, but many of them have to top up their contracted hours with extra working time for free.
"Around one in five workers regularly do unpaid overtime but it's becoming the norm in far too many workplaces.
"Whilst most people have no objection to putting in some extra hours to help their employer through a busy period, an entrenched long hours culture causes stress, health problems and lower morale."
The TUC has declared Friday a Work Your Proper Hours Day, when it is calling on employers to encourage staff to take a proper lunch break and leave work on time.