Spending on locum doctors by hospitals in England has nearly doubled in the past two years, figures suggest.
The Royal College of Surgeons said temporary doctors were now "propping up" the system and linked the rise to EU laws restricting working hours.
The RCS estimated that spending on locums topped £750m last year, after gathering evidence from 96 trusts.
Health Secretary Andrew Lansley agreed it was a problem, as high use of locums could affect patient care.
There have been reports that the European Working Time Directive, which limits the working week to 48 hours and was introduced into the NHS last year, has been harming the health service.
Unions have complained it has damaged the amount of training junior doctors get, while some hospitals have reported difficulties filling rotas.
The RCS believes the rise in spending on locums is a consequence of the rules.
The college asked for data from 164 trusts under the Freedom of Information Act, getting results from 96.
These figures were then extrapolated to give figures for the whole of England. These suggested spending rose from £384m in 2007-8 to £758m last year.
A third of the £758m figure related to surgical posts.
RCS president John Black said: "It seems ridiculous that at a time of economic crisis, with wide-ranging cuts to services across the board, we are seeing astronomical sums of money being thrown at locum doctors in order to prop up services that are only falling apart because of an ill-conceived European law."
Health Secretary Andrew Lansley added: "The scale of increase in the cost of agency staff in the NHS is unacceptable. There is also a practical concern about continuity of care for patients.
"It is clear that the the European Working Time Directive is having an impact on the number of locum doctors."
He said ministers would seek changes to the way it had been implemented in the health service in future negotiations.