Middle managers in France have taken their complaints about the legislation introducing a 35-hour week to the European Court of Human Rights.
If successful the TUC has indicated it may use the case to challenge the remit of the UK law on working time.
The CFE-CGC white-collar union has begun proceedings against the French government claiming that its members' exclusion from the law's limit of 35 hours on the working week is discriminatory and violates their right to a private and family life.
It is demanding that the government pays FFr 78bn (£7.1bn) each year in compensation for the extra hours that the country's one million middle managers will work as a result of the lesser protection the law provides them compared with other employees.
The French legislation - known as the Aubry II Law after employment minister Martine Aubry - came into force in January. While it imposes a 35-hour limit for most employees, middle managers who do not have working hours specified in their contracts are restricted to 217 days - the maximum number of days they can work in a year.
By calculating its members' maximum working time in days, the CFE-CGC claims that not only does the law offer inferior protection compared with that for other workers, but that it could also lead to them working longer hours than in the past. They could be required to work up to 13 hours a day, six days a week, the union argues.
It claims the legislation puts France in breach of the European Convention of Human Rights, by denying middle managers the same rights as other employees to a private and family life.
A victory by the French union could have implications for UK Working Time legislation, according to TUC senior policy officer Sarah Veale, pointing to the similar exclusion of some UK white-collar staff. "If the French case reveals an opening that is available to us, we will use it," she said.