People are working longer hours, reversing a 10-year trend of a cut in the working week, a report suggests.
The TUC is concerned that some people spend too long at work
More than one in eight people now work more than 48 hours a week, rising to one in six in London, the TUC said.
TUC general secretary Brendan Barber said the "disturbing" findings suggest there is "undoubted abuse of the law" by some employers.
Pat McFadden, minister for Employment Relations, said "more employers than ever" were allowing flexible working.
"Long term trends for the amount of people working long hours have been going downwards, with a fall of 20% in the number of people working more than 48 hours since 1997," he said.
Meanwhile, the union organisation said an analysis of official figures revealed that 3.2 million people were now working more than 48 hours a week - more than 13% of the workforce.
This figure was up from 12.8% last year.
According to the study, the biggest rise in the number of people working a 48-hour week was in the south-east of England and London, with 16% of staff in the capital now working long hours.
Mr Barber said: "These are very disturbing numbers. They suggest that the slow, but at least steady, decline in those working more than 48 hours a week has come to an end.
"Many employers recognise that overworked staff are unproductive by introducing more flexibility and better work-life balance, often under union pressure."
He said the efforts of these employers were "being undone by those who don't care about long hours".
"There is undoubted abuse of the law, but employers know they can get away with it because it is rarely enforced," he said.
According to the TUC, official figures underestimate long hours because they are unlikely to include migrant workers or people who live at their place of work, such as hotel or care staff.
A European directive aimed at limiting hours exists, but workers in the UK can opt out, leading to union claims that employers are abusing the system.
Mr McFadden pointed out that "in the UK, people have the choice to refuse to work long hours if they don't want to and the flexibility to work longer hours and earn overtime if they wish.
"Research has shown that seven out of 10 long-hours workers would not want a cut in hours if it meant a cut in pay.
"Other surveys indicate 85% of UK workers said their working hours fit 'well' or 'very well' with their family and social commitments."