EU rules that junior doctors can only work 48 hours a week should be suspended, says a pressure group.
As the UK grapples with swine flu, the pressure on the NHS is mounting and staff going off sick could have a further impact, according to doctors' pressure group RemedyUK.
It is calling for special measures to bypass the reduction in a working week from 56 to 48 hours, which comes into force for junior doctors on Saturday.
Other doctors have been working under the European Working Time Directive (EWTD) for some time.
Individual doctors can opt out of the rules and choose to work longer but there are calls for entire departments to be able to opt out.
The Royal College of Surgeons has also called on the Government to suspend the EWTD if things get worse.
Around 100,000 people a week are currently being diagnosed with swine flu in England, figures published by the Health Protection Agency (HPA) show.
Scotland, Wales and Northern Ireland have been less hit by the virus, which has now touched 160 countries worldwide and caused at least 800 deaths.
RemedyUK, which has 8,000 members, warned that frontline doctors have a high risk of exposure to swine flu as a result of dealing with ill patients.
The impact of sick leave in the winter months calls into question how the NHS will be able to continue to deliver services, it said.
Doctors have been coping well but, from August 1, many will have an extra day off work every week, it said.
Richard Marks, head of policy at RemedyUK, said cash had been spent on setting up the National Pandemic Flu Service for England, which employs people with no medical training.
"Millions have been spent on staff call centres using non-medical staff to diagnose and prescribe but at the same time they are reducing doctors' working week by one full day.
"It's probably the worst time in living memory to do this."
Mr Marks said the introduction of EWTD had been planned for many years but planners could not anticipate how the flu pandemic would alter the way hospitals are working.
He said introducing a change to doctors' working practices at this time seemed an "unnecessary risk".
Matt Jameson Evans, chair of RemedyUK, said: "Unfortunately we have a camel's back situation and swine flu is more of a sledgehammer than a straw.
"We already know most doctors are against EWTD, we just need the leadership to do the right thing here."
John Black, president of the Royal College of Surgeons, said earlier today that the EWTD would put further pressure on the NHS.
He called for Government assurances that the restriction would be suspended if necessary.
"It could fizzle out or we could have a one, two or three-stage serious pandemic," he said.
"If that happens everybody, of course, will work whatever hours are necessary to keep the patients alive in a crisis.
"I trust that if that happens, the Government will not fudge it and they will actually say that the European Working Time Directive leaves no slack at all in the system and if there is a major crisis it should be suspended," he told the BBC Radio 4 Today programme.
The College has been hugely critical of the new rules, which, it says, could lead to "catastrophic" shortages of staff and hit training.
Yesterday, the Government rejected accusations that it should have launched the new swine flu helpline and website earlier.
Tomorrow, a report from the House of Lords Science and Technology Committee is expected to criticise ministers for not setting up the National Pandemic Flu Service in April.
But Health Minister Gillian Merron said: "To say that the National Pandemic Flu service has been delayed, or that it should have been introduced sooner, is untrue.
"The service was set up at the request of GPs and the NHS and has been welcomed by them.
"Launching the service could only be done at the point where we moved from local outbreaks of swine flu to significant levels of infection across the country."
She said the service was activated when the number of PCTs reporting exceptional levels of activity dealing with swine flu jumped to 110.
More than 58,000 assessments were made by the new service on Thursday last week - its first day of operation - which was launched to help relieve the pressure on GPs and the NHS.
Ms Merron said: "When I gave evidence to the House of Lords Science and Technology Committee, peers were interested to hear about the preparedness of the National Pandemic Flu Service which would be, as it has been, activated when it was needed.
"They welcomed this reassurance, the details of which they had been unaware."
Yesterday, Health Secretary Andy Burnham warned that public panic over the outbreak could put extra pressure on the NHS.
He said: "It is very important for everybody to keep a sense of perspective. It has been a mild virus in the vast majority of cases, with relatively mild symptoms from which people recover fully fairly quickly.
"If people are made unnecessarily anxious, it makes the lives of NHS professionals, who are already under enormous pressure, far more difficult as people become unduly worried.
"People should be assured that we have been planning our response to a pandemic for a long time."
He also said swine flu victims were getting Tamiflu "quickly and conveniently" using the National Pandemic Flu Service.
Pregnant swine flu victim Sharon Pentleton is still receiving treatment in Stockholm after suffering a rare and severe reaction to the virus.
The 26-year-old's condition is described as "stable but still critical".
She was flown from Crosshouse Hospital in Kilmarnock, Scotland, to Karolinska University Hospital to be given Extra Corporeal Membrane Oxygenation (ECMO) after she developed adult respiratory distress.
The UK only has five beds for the procedure - all of which were full.
Shadow tourism minister, Tobias Ellwood, said he will hold an urgent meeting to discuss the impact of swine flu on British tourism.
The meeting will be attended by representatives from Visit Britain, Visit England, Visit London, The Tourism Alliance, the British Hospitality Association, the travel industry and the health service.
He said: "We are already seeing some evidence of British tourism being affected by headlines about the swine flu pandemic.
"It is therefore essential to develop a strategy to ensure both domestic and overseas visitors recognise that Britain remains a safe place to visit and we are very much open for business."
Shadow health minister Stephen O'Brien said: "We warned Labour ministers repeatedly that their failure to negotiate an opt-out for junior doctors could compromise NHS services and potentially undermine patient care.
"Now, with the advent of swine flu, the need is even greater.
"We urge the Government to listen to those on the frontline - including RemedyUK and the Royal College of Surgeons - who are facing undue pressure, and take full advantage of any allowances available under the directive."
Dr Andy Thornley, chairman of the BMA's junior doctor committee, said: "Clearly pandemic flu is going to place additional pressure on an NHS that is trying to adapt to the introduction of the 48-hour week for junior doctors.
"The Government needs to be much clearer in communicating how it plans to deal with these additional pressures as it is unacceptable that so little information is trickling down to junior doctors.
"It is also important that the NHS works hard to reduce unnecessary bureaucracy and inappropriate work so that junior doctors can do what they do best - treat their patients.
"Abandoning the working time directive days before it's due to be implemented would be inappropriate given that most of the extra work is currently being done by colleagues in general practice."
A spokesman for the Department of Health said: "Doctors, nurses and other healthcare staff can work longer hours when they need to.
"During national emergencies there are special provisions and flexibility within the regulations for emergency situations.
"Medical directors have plans in place to ensure NHS organisations are able to meet the needs of patients and that the hours doctors and other healthcare staff work are balanced over a period.
"Medical directors will carefully review the local situation as the current pandemic flu outbreak continues."