A large proportion of operations carried out at night involve either junior anaesthetists or surgeons, says a major report.
Consultants cannot attend every operation
In 45% of night-time operations, patients were anaesthetised by a non-consultant doctor - and in one in five, the surgeon was a junior too.
Some "juniors" are highly experienced, but ministers say they want fewer operations unsupervised by consultants.
The report was published after a study of more than 73,000 procedures in 2002.
While the idea that non-consultants could anaesthetise or operate on patients might be alarming, some of the doctors involved are specialist registrars - just below consultant grades in terms of experience.
Many have been covering the same specialty for years, in preparation for taking a consultant job when one becomes available.
The sheer number of consultant anaesthetists and surgeons means that, short of vastly increasing their working hours, it is presently impossible to provide round-the-clock consultant cover at most hospitals.
However, in many instances, the only cover available is from lower grades of junior, such as senior house officers (SHO) who have only been in the job a few years.
At night, during one week sampled by the researchers from the National Confidential Enquiry into Perioperative Deaths (NCEPOD), more than a third of hospitals offered SHO cover in anaesthetics.
More than one in 20 had only SHO grades covering as surgeons.
These figures are an improvement from the last time this survey was carried out, in 1995/6, but the report's authors say that further improvements are needed.
Dr Anthony Gray, of NCEPOD, said: "While it must be understood that many patients who require emergency operations at night are likely to be very sick, we still have the situation that in almost half the cases, they will be anaesthetised, and in a fifth of cases, operated on by relatively inexperienced doctors.
"We strongly recommend a further improvement in consultant supervision out of hours."
Changes to the rules governing how long junior doctors can work mean that many more consultants are involved in emergency and out-of-hours work.
Dr Gray said: "As an ironic result, they are often far more fatigued than their junior colleagues."
Health minister Lord Warner said that the report suggested that operations were becoming safer because of extra staff recruited in the past few years.
He said: "But as the report highlights, there are still too many emergency operations involving junior grades of medical staff without the supervision of a consultant, and we will continue to invest more resources, work with staff and review working patterns to make sure that further progress is made."