Patients are put at risk when junior doctors start work, the NHS medical director conceded as he outlined measures to end the so-called health service “killing season”.

Young medics will now have to shadow their senior colleagues as part of a scheme designed to counter a spike in death rates observed each August as they begin their jobs.

Sir Bruce Keogh said all first-year doctors will spend a minimum of four days in a mandatory shadowing role before they take up their positions this year.

It is hoped the initiative will halve the number of errors made in the early stages of their careers.

Under the current system, people admitted to English hospitals in an emergency on the first Wednesday in August have, on average, a 6 per cent higher mortality rate than those admitted on the previous Wednesday.

The figures - supported by Dr Foster Intelligence, a body which tracks death rates - have led to this period being dubbed the “killing season”.

The day newly qualified medical graduates start their jobs has become known as “Black Wednesday”.

Sir Bruce said: “There is some evidence of increased risk to patients as new doctors take their first steps.

“So, learning from pilots across the country, we’ve agreed that all new first-year doctors should undertake a period of paid shadowing at the end of July, starting this year.”

Trials in Bristol showed mistakes made by new doctors in their first four months were reduced by more than 50% after a week of shadowing and targeted teaching.

The Department of Health said: “Our aim is to ensure that all junior doctors spend a minimum of four working days shadowing the job that they will be taking up and completing trust-based induction.

“This will help them to become familiar with their new working environment and include a handover of their clinical responsibilities.

“Evidence from the pilots suggests that shadowing can reduce the number of serious adverse events.”

Sir Bruce has written to final-year medical students, medical school deans, postgraduate deans, strategic health authority chief executives and trust chief executives to explain the scheme which will be introduced nationally for the first time this year.

A voluntary programme is already in place offering junior doctors the option to shadow experienced medics.