Doctors have warned hospitals across the UK are unprepared for a cut in junior doctors' hours due next year.
Under the European Working Time Directive (EWTD) the cap on juniors' hours will fall from 56 hours to 48 hours a week in August.
But the British Medical Association fears funds to help hospitals prepare are not reaching the front line.
However, the Department of Health said good progress towards the targets had been made in some areas.
The government has set aside £110m this year to help hospitals in England prepare for the reduction in junior doctors' hours, with another £310m due to come on steam in 2009.
The cash is supposed to be spent on employing extra staff, or on new systems to help doctors prioritise calls more effectively.
But the BMA is concerned that primary care trusts have not handed out the money, or spent it in other ways.
It is also concerned that some trusts were lagging behind in their preparations for the change.
Figures from April this year showed 46% of junior doctors were on rotas which would be unlikely to comply with the new rules.
The BMA has written to health minister Ann Keen to express its concerns.
The text of the letter says: "There is a certain amount of anecdotal evidence that acute trusts are unaware of this additional funding, and those that are aware have found it difficult to secure.
"As these funds were not ring-fenced there are suspicions that the funding has been spent on other initiatives."
Dr Jonathan Fielden, chairman of the BMA's consultants committee, said: "This major change to junior doctors' hours is probably the biggest challenge hospitals will face in 2009.
"It will need focused consultant expansion and change towards consultant-based care to maintain quality of care.
"Hospitals need to be fully prepared as early as possible to minimise disruption to services and maintain high quality patient care.
"It's crucial that funding to help them is protected, and that it reaches the frontline."
Dr Andrew Thornley, chairman of the BMA junior doctors committee, said the 48-hour limit would have a big impact on both patients and junior doctors.
He said: "It is vital that trusts are properly resourced to ensure that changes don't have a negative impact on either the quality of care, or on training."
A Department of Health spokesperson said significant progress had already been made, with some trusts already operating a 48-hour week rota.
However, the spokesperson added: "We are monitoring the situation as some smaller specialties and isolated hospitals may find meeting the deadline more challenging."
In Scotland, a survey carried out in November found 49% of junior doctors were currently employed on rotas that exceed the 48 hour limit.
Dr Peter Terry, chairman of the BMA in Scotland, said: "It is disappointing that we have known about this change for more than a decade, yet it looks like NHS boards are nowhere near prepared to deal with it."
A Scottish Government spokesperson said ministers were committed to ensuring that the EWTD was fully implemented, and progress was being closely monitored.
The BMA in Wales is also investigating the issue, amid concerns that many trusts are not up to speed.
Additional money has been made available in both Wales and Northern Ireland to aid preparations.