There is "clear" evidence that the standard of NHS care varies through the week, the Department of Health has said.
It was responding to reports that an internal memo had questioned whether putting more consultants on wards at weekends would improve patient survival rates.
Health Secretary Jeremy Hunt has previously referred to research that suggests up to 11,000 more deaths happen each year among patients who are admitted on weekends.
The Government hopes to improve the standard of weekend services by ensuring more staff are working.
However The Guardian claimed that a departmental update on the proposals circulated in mid-January said the DoH "cannot evidence the mechanism by which increased consultant presence and diagnostic tests at weekends will translate into lower mortality and reduced length of stay".
The document reportedly said more than 11,000 new staff will be needed to function identically to weekdays, including 3,000 nurses and 4,000 doctors, requiring an extra £900 million each year.
It also raised concerns that community and social services would not be able to cope with increased discharges at weekends.
A Government spokesman insisted that its plan was supported by the research.
The spokesman said: "There is clear, independent clinical evidence of variation in the quality of care across the week and working together with the NHS we are determined to tackle this problem. Making sure the right staff and support is available for all patients seven days a week is a key part of our approach."
However proposals that could include new contracts being forced on consultants and junior doctors have been met with resistance.
The British Medical Association said doctors and nurses already worked "around the clock, seven days a week, and do so under the existing contract".
Mark Porter, BMA chairman of council, said: "This leaked document makes clear that more seven-day services will require not only thousands of extra doctors, nurses and support staff but an additional investment in both the NHS and community care.
"Its findings also show no proven link between weekend mortality rates and consultant presence, and suggests that other investment is more necessary.
"It also echoes the BMA's concerns around the Government's recruitment target for GPs, at a time when one in three GPs are considering retiring in the next five years, and hundreds of GP trainee posts were left vacant this year.
"If the Government is to continue with its plans for extra seven-day services, it owes it to patients to convincingly explain how it will finance and staff it."