Doctors' leaders have said they will back any juniors who want to take legal action over working too many hours.
Junior doctors will be able to take legal action if they work too long
From Sunday, under the European Working Time Directive, no junior should work more than 58 hours a week, including the time they spend on call.
The British Medical Association has said it will support legal action by any doctor who is forced to work more hours.
Other UK workers are limited to 48 hour weeks under the directive.
They have been covered by the rules since 1998.
Junior doctors have so far been excluded because of the difficulties of introducing the hours restrictions for them.
Their hours will not have to fall to 48 per week until 2009.
The directive also sets new minimum rest requirements for doctors in training.
However research by the NHS Confederation last month showed that a sixth of trusts did not expect to meet Sunday's deadline for a 58 hour week.
The BMA has said it was ready to "name and shame" NHS trusts which failed to meet the new limits , because they would mean the quality of doctors training would be affected.
Some hospitals have used the opportunity to cut hours to introduce fresh approaches to working patterns which have driven up standards, the BMA said.
But it added that the majority of hospital have simply brought in shift systems which require doctors to work for stretches of up to 13 hours.
The BMA said doctors' training if these shifts take place at night, because juniors would miss out on seeing procedures they need to learn about which only take place during the day.
Hospitals could be fined up to £5,000 for each breach of the rules by the Health and Safety Executive and be taken to employment tribunals by junior doctors.
Simon Eccles, chairman of the BMA's junior doctors committee, said: "This is health and safety legislation - it's being introduced to protect patients as well as doctors, and hospitals need to take it seriously.
"Where the directive is flouted, the BMA will provide doctors with information, advice and, where appropriate, legal support."
He added: "We shouldn't forget that junior doctors are in hospitals to learn as well as look after patients. Safe hours must not mean unsafe training."
A Department of Health spokesman said: "The NHS isn't complacent about the Working Time Directive and a lot of work has been done to help hospitals implement the directive.
"It has been working closely with the BMA and other stakeholders for some time."
He added: "The vast majority of trusts will be ready on August 1. A handful have teething problems securing compliance in a small number of specialties, but patient care will not suffer."
Beverly Malone, general secretary of the Royal College of Nursing, said: "We believe that expanding nursing roles offer a better way in safeguarding patient care.
"Advanced experienced nurses are performing anaesthesia, working as first assistants in theatres and nurse practitioners operate as senior house officers, providing continuous patient care."
Paul Burstow MP, Liberal Democrat health spokesman, said: "The threat of legal action should come as no surprise to ministers who have long known and long been warned of the impact the Working Time Directive would have on the NHS.
"Successful court actions, at £5,000 a pop, will simply be more money out of NHS coffers, leaving even less to treat patients."