Given the culture of long hours in the profession, it will come as little surprise to many that the majority of law firms have or will be asking their staff to sign the opt-out of the Working Time Regulations that became law on 1 October last year.

More surprisingly, one-in-four firms admitted that staff who refused to sign the opt-out would be damaging their partnership prospects.

The law clearly states that any worker "being subjected to detriment" for refusing to work more than 48 hours has the right to pursue a claim through an industrial tribunal against his/her employer.

The same right applies to anyone actually forced to work over the limit.

The other interesting revelation is that a small but significant number of firms – 6% – believe the directive does not apply to them.

One of the exemptions is employees whose "work is not pre-determined or measured" or who can control when and how work is to be done.

The Department of Trade & Industry's interpretation is that this only covers managing executives, those employed by a member of their family and vicars and rabbis.

But some City firms, including Clifford Chance, consider that all of their solicitors are managing executives and therefore exempt. "Looking at the legislation and the way solicitors organise their work, we think that is the case," says head of personnel Alisdair Dawson.

"But we will monitor the situation and we may have to change our interpretation."

Michael Jones, a partner at niche employment specialists Jones & Warner is very sceptical.

"How on earth law firms can put all their solicitors in that category is beyond me," he says. "I think there will be a test case soon."

A useful comparison is the civil service, whose lawyers advised them that all but permanent secretaries and the top-tier of management were subject to the regulations.

Responsibility for monitoring the situation lies with the Health and Safety Executive. It has the power to investigate any company if it receives a complaint and is expected to take a proactive approach to enforcement.

Whether or not firms are leaving themselves open to test cases, there are longer-term implications.

The working time directive is just one piece of a jigsaw of legislation emanating from the European Social Charter. The Government's White Paper Fairness at Work will grant parental leave and part-time worker rights by April next year.

In not complying with the spirit of the law and EU policy, some law firms may be storing-up trouble for the future.

"The City firms' approach will probably do for the time being," Jones says.

"But there is an extraordinarily high degree of dissatisfaction among the younger members of the profession about how they are expected to work. In future, solicitors may look to the social chapter and other regulations for protection.

Firms are creating a rod for their own backs."