Emergency patients are more likely to die in NHS hospitals when their admission coincides with an influx of newly qualified doctors, a study has shown.

Death rates for patients brought into English hospitals on the first Wednesday in August were 6% higher than for those arriving a week earlier, researchers found.

The first Wednesday in August is traditionally when newly qualified junior doctors take up their positions in NHS hospitals in England.

Researchers from Imperial College London said the “statistically significant” trend emerged after factors such as age, sex, social and economic background and additional diagnoses were taken into account. Mortality was higher for medical patients than those requiring surgery or suffering from cancer. For medical patients, death rates increased by 8%.

The study was unable to draw firm conclusions about what had led to the extra deaths. But the researchers said it was a widely held belief that early August is an “unsafe period” to be admitted to NHS hospitals, since this was when newly qualified junior doctors start work.

Dr Paul Aylin, from the Dr Foster Unit at Imperial College London, said: “We wanted to find out whether mortality rates changed on the first Wednesday in August, when junior doctors take up their new posts. What we have found looks like an interesting pattern and we would now like to look at this in more detail to find out what might be causing the increase.”

The researchers studied 299,741 patients who were admitted as emergency cases to hospitals belonging to 175 NHS Trusts between 2000 and 2008.

Over the nine-year period a total of 151,844 patients were admitted on the last Wednesday in July, and 147,897 on the first Wednesday in August.

Both groups were monitored for a week and the number of in-hospital deaths recorded. A total of 2,182 patients died who were admitted on the last Wednesday in July, compared with 2,227 of those brought into hospital on the first Wednesday in August.

Hospital death rates normally fluctuate throughout the year, peaking in winter. But the study — the largest of its kind ever conducted — showed a relatively consistent pattern.

Dr Shree Datta, who chairs the British Medical Association's Junior Doctor Committee, said: “This study has to be judged alongside many previous studies looking at mortality rates before and after junior doctors start their new jobs, which have not shown any differences. Clearly even a small increase in death rates is of great concern and we need further research to see whether this is a real effect or an anomaly.”