Patients are more likely to die if admitted to NHS hospitals on weekends, a new report claims.
The study found that around 11,000 more people die every year within 30 days of admission to hospital on Friday, Saturday, Sunday or Monday compared with other days of the week.
The findings - blamed partly on a lack of consultant cover and "reduced" services - were described by Health Secretary Jeremy Hunt as a "wake-up call".
It found that, even when taking into account the fact that patients admitted on weekends tend to be sicker, people remained 10% more likely to die if admitted on a Sunday compared to a Wednesday.
For Saturday admissions, the likelihood was found to be 7%, the study claimed.
Writing in the British Medical Journal, the experts from University Hospital Birmingham NHS Foundation Trusts and University College London, said there was a "clear association" between weekend admissions and worse outcomes for patients.
They argued that services "inside and outside the hospital" are reduced on weekends, despite the fact there were more emergency admissions on Saturdays and Sundays.
The "weekend effect" was also being felt on Fridays and Mondays, they claimed, as services were often reduced from late Friday, "leading to disruption on Monday morning".
Mr Hunt, who plans to impose seven-day working on hospital doctors in England, said the research may show worse problems than expected.
Dr Mark Porter, head of the British Medical Association, said doctors wanted high standards of care seven days a week, but claimed action on this had been "undermined" by calls for the entire NHS to move to a seven-day system "without any clear prioritisation".
He called for additional investment in nursing, diagnostic and support staff as well as the extra doctors if urgent and emergency care were to move to a seven-day basis.
NHS medical director Sir Bruce Keogh, who worked on the study, said it was unacceptable that the way services were organised could appear to cost lives.
The Royal College of Surgeons president, Clare Marx, said: "It is clear that patients that need treating at the weekend are less likely to be seen by the right mix of junior and senior staff, and experience reduced access to diagnostics.
"Many doctors and NHS staff already work at nights and weekends and they should be valued and thanked for continuing to provide care during those unsocial hours.
"However, the evidence shows that this is not currently standard practice even in high risk emergency care. This has to change."