People are more likely to die if they are admitted to hospital at weekends, a large-scale review of NHS data suggests.
Patients are 16% more likely to die if they are admitted on a Sunday than a Wednesday, and 11% more likely to die if they are admitted on a Saturday.
For every 100 deaths following admissions on a Wednesday, 116 occur for admissions on a Sunday - a "significant increased risk", the researchers said.
It follows a report in November which found patients needing emergency care are almost 10% more likely to die if they are admitted on weekends and are less likely to receive prompt treatment.
The new analysis covered all admissions - more than 14.2 million - to NHS hospitals in England during 2009/10, including both emergency and planned admissions.
It looked at more than 187,300 patient deaths within 30 days of being admitted to hospital.
The researchers found higher death rates if patients went in on a weekend but a slightly lower death rate if people were already in hospital on a weekend.
Being already in hospital on a Sunday led to an 8% reduced risk of dying on that day compared to already being in hospital on a Wednesday.
The medical conditions resulting in the biggest number of in-hospital deaths included pneumonia, congestive heart failure, heart attack, septicaemia, acute renal failure, urinary tract infections and neck or hip fracture.
The experts, including from University College London and the Universities of Birmingham and East Anglia, wrote in the Journal of the Royal Society of Medicine (JRSM): "We identified a significantly higher risk of subsequent in-hospital death during the 30-day follow-up period associated with admission during the weekend (Saturday or Sunday), compared to mid-week days.
"Admission on Tuesday through Friday was associated with the lowest risk of in-hospital death, while admission on Sunday was associated with the highest risk.
"Admission on Saturday was associated with a marked increased mortality (death) risk and admission on Monday was associated with a less, but statistically significant, increased risk."
Lead researcher Professor Domenico Pagano, from the University Hospital Birmingham Foundation Trust, added: "These results offer conclusive evidence that confirms previous reports of increased 30-day mortality risk for patients admitted to hospital with emergency conditions at the weekend compared with the rest of the week."
He said several reasons may be behind the findings, including that patients who are seriously ill can find themselves admitted on weekends.
If they were less ill, they would have had their admissions postponed until a week day.
Prof Pagano also said reduced staffing and fewer senior doctors on duty as well as poor access to diagnostic tests on weekends could have an effect.
His team said seven-day access to "all aspects of care" could improve the outcomes for higher-risk patients currently admitted on weekends.
On Sunday, Health Secretary Andrew Lansley suggested more could be done to boost the number of senior doctors working weekends.
Mr Lansley told the Sunday Telegraph: "By opening some services seven days a week, more patients will get the care and treatment that they need when they need it.
"In some parts of the NHS, this is already happening. We will work with all the professional associations to encourage progress on this across the NHS.
"By increasing the number of consultants who are on duty at the weekends, we will both strengthen the training of junior doctors and improve quality of care for patients."
Last November, the Hospital Guide, from Dr Foster Intelligence, found that around one in eight trusts had higher-than-expected death rates on Saturdays and Sundays.
It said hospitals with the fewest senior doctors on duty have the highest death rates overall.
In a "handful" of trusts, the death rate rose by 20% or more at weekends.
Previous studies have also found the risk of dying in NHS hospitals on weekends and bank holidays is higher than during the week.
Mr Lansley added: "It is unacceptable that patients admitted to hospital on a Saturday or Sunday stay longer and have worse results.
"Much of the rest of the country continues to be open for the public's needs at weekends - an NHS that revolves around patients should be the same."
The Department of Health is looking at ways the NHS could provide more services at the weekend, including more tests and consultant cover.
While this will not be imposed, tools such as providing financial incentives for trusts to promote weekend working and incentives for GPs to strike deals with hospitals which deliver care at weekends could be introduced.
As well as publishing data on outcomes for patients, hospitals may be rewarded a small bonus percentage of their income for offering care at weekends.
Dr Andrew Goddard, director of medical workforce at the Royal College of Physicians, said: "This study is further evidence that patients admitted at weekends are more likely to die following admission than patients admitted to hospital during the week.
"There are many reasons for this, but the two most important are that the patients are more ill and there are fewer doctors available.
"The Royal College of Physicians has already called for any hospital admitting acutely ill patients to have a consultant physician on-site for at least 12 hours per day, seven days a week."
NHS Confederation deputy chief executive, David Stout, said: "This research demonstrates that there is an inconsistency in the care patients receive dependent on day and time of admission.
"There is no doubt that this is something the NHS has to improve."