Junior doctors say they may sue hospitals which breach a new agreement limiting their working hours.
Junior doctors can now no longer work more than 56 hours a week
Under new rules coming into force on Friday, junior doctors are no longer allowed to work more than 56 hours a week, or 72 hours including time on call.
But three out of four hospitals are failing to comply, the doctors' representative body, the BMA, warned earlier this week.
And some hospitals which are apparently obeying the rules are merely fiddling figures, it said.
Spokeswoman Dr Jo Hilborne told BBC News that many were using "hastily-drawn up rotas that are not easy to work, and do not provide good training and education".
The rotas were being "put into place hurriedly at the last minute in an attempt to bring working hours into compliance".
Some hospitals were even asking doctors to lie about their working hours to help managers meet the target, she said.
"We do have reports of trusts putting pressure on juniors to falsify their monitoring, so they appear to be working a compliant working pattern when in fact they're not."
The BMA said it was "examining avenues of legal redress" for doctors working outside the limits.
It could mean the NHS hospital trusts involved ending up in court for breach of contract.
Some NHS trusts have warned the reduced working hours could result in a shortage of cover, especially in accident and emergency departments.
But healthcare workers say employers have known about the obligations for a long time, so there should be no excuse for them not being prepared.
The root of the problem, Dr Hilborne said, was in continuing staff shortages.
"There are not enough doctors in the UK, notwithstanding the recent increase in figures, compared to other European countries," she said.
"At the end of the day, if you're going to reduce the number of hours any particular doctor works, really the only sensible way to do that is either to increase the number of doctors... or look at new and different ways of doing that work."
Tighter rules on way
One possible solution is giving senior nurses the responsibility to take on some aspects of the doctors' workload.
Such a scheme has been successful at Great Ormond Street children's hospital.
The BMA said it improved patient care and junior doctors' morale.
The government says compliance with the new rules is a matter for individual trusts.
It says there has been good progress in recent years, and that 80% of junior doctors are now working within the agreed limits.
The guidelines on working hours were first drawn up in 1991, but have only now become contractual for all junior doctors.
There will be pressure to reduce their hours still further next year when the European Working Time Directive comes into force.
That will limit all junior doctors to working no more than 58 hours a week from August 2004.