A majority of NHS hospitals have not planned or budgeted for a move to a seven-day health service, according to new figures.
Four out of five NHS trusts have not calculated staffing levels or costs associated with weekend working, according to Freedom of Information requests obtained by Sky News.
Those which had made plans warned millions in extra funding would be needed, along with more staff, to make the necessary changes.
The Government has made extending more hospital services across seven days a priority but the doctors' union and health leaders have criticised the plans.
New contracts for consultants under negotiation but Health Secretary Jeremy Hunt has warned he could impose new terms if an agreement cannot be reached - a threat which has prompted the British Medical Association (BMA) to ballot junior doctors on industrial action over "unsafe and unfair" trainees' contracts.
Dr Paul Flynn, chairman of the BMA's consultants' committee, told Sky News hospitals were unsure which services they would have to provide and would need more funding.
"You can't take the work we are doing over five days and spread it over seven. That's simply not going to work," he said.
"Hospitals are under considerable pressure at the moment just to provide current services.
"It was never credible to think that we could have a step-change in the quality of what they were doing and also stay still in terms of finances."
A total of 119 of the 155 NHS hospital trusts in England responded to the broadcaster, with 62 saying they had made no preparations for increasing weekend services.
Some 35 said they were beginning to calculate costs and staffing needs, while 22 said they already provided a full service at weekends or had worked out how to do so.
Great Western Hospitals NHS Foundation Trust in Swindon estimated it would need 14 more consultants and £7.2 million to extend its services, while Epsom and St Helier University Hospitals said its total bill would be £8.5 million, with 34 more consultants required.
Figures released this month revealed patients admitted to NHS hospitals on weekends are more likely to die than if they are admitted during the week, with Mr Hunt calling the data a "wake-up call".