A new study reveals that workers are putting in longer hours, reversing a 10-year trend of a cut in the working week.
The TUC said more than one in eight people now work more than 48 hours a week, rising to one in six in London.
The union organisation said an analysis of official figures showed that 3.2 million people were now working more than 48 hours a week, over 13% of the workforce, up from 12.8% last year.
The figures showed that a "hard core" of bad employers are taking no notice of the law or calls to give staff a better work-life balance, said the TUC.
The biggest increase in the number of people working a 48-hour week is in the South East and London, with 16% of staff in the capital now working long hours, the study found.
TUC general secretary Brendan 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.
"There is undoubted abuse of the law, but employers know they can get away with it because it is rarely enforced. Neither the Health and Safety Executive nor local authorities who share responsibility for enforcement have the resources to implement the law."