Hospitals across the UK could face legal action if they do not recognise the rights of junior doctors to work fewer hours.
The British Medical Association (BMA) is examining legal options open to doctors whose rights are being ignored by three-quarters of the country's hospitals.
Maximum hours, standards of accommodation and minimum rest requirements were agreed in 1991. Trusts who do not comply could have training posts taken away.
Under the terms of the 1991 New Deal, junior doctors should not be on their feet working for more than 56 hours a week, or do more than 72 hours of total work (including time spent on call).
The limits were initially introduced as guidance, and have applied to first-year doctors since 2001, but they will now be contractually binding for all junior doctors.
Jo Hilborne, joint deputy chairman of the BMA's Junior Doctors Committee said: "Despite the fact that hospitals have known about the deadline for 12 years, only a quarter have done enough to meet it.
"Nobody wants to see doctors being forced to take legal action, but because of the lack of preparation in some trusts, it could happen," she said.