Hospital patients are almost 10% more likely to die on Saturday or Sunday than during the week, a report has revealed.
Research company Dr Foster concluded that weekend treatment is "risky" after finding a 10% spike in hospital mortality rates, across 147 hospital trusts.
One in eight trusts had higher than expected death rates on Saturdays and Sundays, while a "handful" of trusts saw mortality rates rise by as much as 20% at the weekends.
The overall death rate for emergency admissions rose from 7.4% on weekdays to 8.1% at weekends, an increase of 9.5%.
Roger Taylor, director of research at the group, said the problem was largely down to staffing issues.
He told the BBC: "It's about having the more experienced staff in the hospital, looking after patients out of normal working hours.
"The junior doctors, they're always around, but they're not the ones making a difference here."
He urged hospitals to make sure they always had sufficient cover so that patients got to see the right doctor.
However, the review, published in the group's Hospital Guide, also suggested that some of the deaths were due to the fact that community services were also reduced at the weekends, resulting in more terminally ill patients being admitted to hospital at that time.
NHS medical director Prof Sir Bruce Keogh said that mortality rates in the NHS are going down, overall, but hospitals with high rates and poor outcomes in the evenings and at weekends need to be investigated.
"I will be asking the NHS medical directors to look closely at weekend services to ensure patients admitted at weekends receive the same standards of care as those during the week.
"This problem is not unique to the NHS, it confronts all health systems in the world, but I am confident the NHS is well placed to address these issues," he said.