Andrew Hobart was not surprised by the finding

Junior doctors are supposed to work a maximum of 56 hours a week, but the British Medical Association says one in four junior doctors is working over this limit.

Although the study itself draws no conclusions as to why doctors make so many mistakes, Dr Andrew Hobart, head of the BMA's junior doctors committee, said there were dangers in current working conditions.

"Long hours and lack of sleep is not good for the patients or the doctors themselves," he said.

"We want to bring hours down to some sort of reasonable level so that doctors are less tired and give patients the care they deserve."

Disasters averted

Rachel Raymond, a business psychologist, carried out the research at a London teaching hospital.

She questioned 78 senior house officers - middle-ranking junior doctors who are responsible for much of the day to day work in hospitals - with an average age of 28 and two thirds of whom were men.

Most had been qualified for a relatively short period - between two and three years - although some had been working on the wards for up to 13 years.

Ms Raymond told BBC News Online that mistakes were common, but not all cases had resulted in disaster.

"There were instances that did have an adverse effect - and some had the potential to go very wrong - but some were averted," she said.

Accepting blame

Most doctors blamed themselves for the mistakes, although one in 10 blamed patients and one in five blamed the government.

A spokesman for the Department of Health said: "We are committed to tackling junior doctors' hours and working conditions, especially measures to improve rest breaks.

"We will be providing another 7,000 doctors over the next three years on top of the 2,000 recruited in the government's first years in office."