Government attempts to curb excessive working hours are not being enforced, with too many employers flouting the law, the TUC has claimed.
The TUC claims a hardcore of bad employers are ignoring working time laws, believing this is the one area of employment law they can break without fearing the consequences.
To support this, unions point to a rise in the number of people working more than 48 hours a week, arguing the long-hours culture is on the increase after ten years in decline.
Brendan Barber, TUC general secretary, said: "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.
"And the government knows that employers can abuse the opt-out as ministers consulted on ways to close loopholes in 2004, but have yet to bring forward any concrete proposals for change."
Mr Barber said the government must close the loopholes that have made the option of voluntary opt-out a "joke".
The TUC claims a quarter of people who opt-out of the European Working Time Directive have no real choice.
Unions are also calling on the government to include working time rights in the current discussions on protecting vulnerable workers.
Research by the TUC finds more than one in eight workers put in more than 48 hours a week, rising to one in six among Londoners.
Unions accept the government and progressive employers are more concerned about the need for a good work-life balance but say a hardcore of bad employers are resistant.
The latest Labour Force Survey found 93,000 more people are now working over 48 hours a week, with 3,242,000 people - or 13.1 per cent of the workforce - working long hours.
Mr Barber continued: "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. But it now looks as if their efforts are being undone by those who don't care about long hours."
The government maintains the number of people working long hours is in decline, with employment minister Pat McFadden pointing to a 20 per cent fall since 1997.
Mr McFadden also said more employers are embracing flexible working, with 91 per cent of employers approving requests granted last year.
He said: "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 ten long-hours workers would not want a cut in hours if it meant a cut in pay."