Priority must be given to urgent and emergency care in plans to boost seven-day hospital services, doctors' leaders have said.
The British Medical Association (BMA) urged the Government to outline its plans for seven-day services including how it will fund and staff them.
The call came after it was reported that some hospitals have 10 times as many consultants handling emergencies on a weekday as on a weekend.
The findings of the High-intensity Specialist Led Acute Care project, which is helping evaluate the drive for more services, were reported in The Sunday Times, which said that at better-staffed trusts there was a ratio of about two consultants attending medical emergencies on a Wednesday compared with one on a Sunday.
However, one trust had a ratio of 16 consultants on a Wednesday compared with one on a Sunday, and another had a ratio of 10.
On average, there were 3.6 times as many specialists attending acutely-ill patients on a Wednesday compared with a Sunday, the newspaper reported.
NHS England medical director Sir Bruce Keogh wrote in an article for the newspaper: "Our weekend services are fuelled by professionalism and goodwill rather than good NHS design. This is not sustainable."
Dr Mark Porter, BMA council chair, said: "It is wrong to claim that the contract for NHS consultants is in any way a barrier to doctors providing urgent and emergency care to their patients seven days a week.
"Consultants do not have the right to opt out of caring for such patients and indeed there is a contractual provision requiring them to do so, which they accept as a professional obligation.
"The BMA has been clear in its support for more seven-day hospital services, with priority being given to urgent and emergency care as the first step towards ensuring that the most seriously ill patients have access to the best possible care whenever they fall ill.
"NHS staff are already providing a first-class service, every day of the week. To say otherwise is simply wrong."
He added: "Sir Bruce Keogh is right when he says it is the professionalism of doctors that sustains current emergency care provision despite the NHS never having invested properly in it as a national priority. And where individual hospitals have made changes, such as at Salford Royal, doctors are providing the backbone of the service.
"The BMA has repeatedly called on the Government to outline their plans for seven-day services including how they will fund and staff them, and the HiSLAC study supports the BMA's call for urgent and emergency care to be made the priority."