Death rates for emergency patients jump 6 per cent when newly qualified doctors start work, according to the largest ever study of hospital mortality rates.

The traditional first day for NHS doctors is the first Wednesday in August. Researchers found that patients brought into hospital the week before were more likely to survive.

Other factors such as gender, age, economical and social background and extra diagnoses were considered but the “statistically significant” trend could not be ignored.

Researchers could not find a definite reason for the higher mortality rate, but said early August was known as an “unsafe period” in hospitals due to the influx of new doctors.

The study found a relatively consistent pattern of NHS hospital deaths, despite the usual seasonal fluctuation and winter peak. In medical journal PLoS One, its authors said: “We suggest further work to look at other measures such as patient safety, quality of care, process measures or medical charge review to identify preventable deaths rather than overall early mortality to further evaluate the effect of junior doctor changeover.”

Patricia Hamilton, the Department of Health’s director of medical education, said: “Local hospitals must ensure that they responsibly manage the introduction of new junior doctors each August by providing appropriate senior cover and supervision.”