Surgery taking place
People who have surgery towards the end of the week are more likely to die than those who have procedures earlier on, researchers say.
A British Medical Journal report into non-emergency operations in England, suggests the overall risk of death from such planned procedures remains low.
But it shows "unacceptable" variation in survival rates through the week, a leading body of UK surgeons says.
The government says it is committed to safe care for patients at all times, reports the BBC.
Researchers from Imperial College London gathered data from all non-emergency surgery undertaken by the NHS in England in 2008-11.
Looking at some 4 million operations they found more than 27,000 deaths within a month of surgery, putting the average risk of death at 0.67%.
The researchers say they are concerned about the significant variation over the week, with the risk lowest for surgery carried out on Monday and then increasing with each subsequent day to peak at the weekend.
The paper shows people who have their operations on Friday are 44% more likely to die than those who have a procedure on Monday.
Professor Antony Narula, of the Royal College of Surgeons, who was not involved in the research, said: "It is not acceptable that there should be such a wide variation in the mortality rates following elective surgery, according to the day of the week the operation takes place."
The researchers say the findings may be due to a poorer quality of aftercare at the weekend, when people who have their surgery later in the week need it most.
Dr Paul Aylin, lead author of the study said: "The first 48 hours following a procedure is most critical and when things can go wrong, such as bleeding and infections. If you don't have the right staff, this is likely to contribute to things being missed.
"If I were a patient I would take comfort from the fact the overall death rate is low, but if I were to have an operation towards the end of the week I would be interested in whether the hospital had the appropriate services to look after me throughout my recovery, including at the weekend."
He says the higher risk of death could be due to fewer doctors, nurses and many non-clinical staff being available on Saturdays and Sundays.
And the study suggests the risk of dying was higher still for surgery conducted over the weekend - 82% greater than on Monday - though the researchers caution only a minority of planned operations currently take place on Saturdays and Sundays.
Previous research looking at emergency procedures shows people are at greater risk of death if admitted during the weekend.
But this is the first large study to look at planned operations, ranging from high-risk procedures such as heart bypass grafts to routine hernia repairs.
Katherine Murphy, chief executive of the Patients Association, said: "Whilst this research once again highlights the NHS's weekend malaise and makes for concerning reading, it unfortunately doesn't identify a new problem, but rather a failure to address an issue that has repeatedly been highlighted in the past...
"It is a shame that, despite the publication of hundreds of pages worth of reports and recommendations, so little action has been taken to actually address the problems identified."
Sir Bruce Keogh, medical director of NHS England, said: "We have established a forum to develop viable financial and clinical options to help our NHS provide more comprehensive services seven days a week."