The new evidence, to be presented to the inquiry on Wednesday, represents a far wider probe than that undertaken by the General Medical Council, which disciplined three doctors from the hospital.

The audits, carried out by cardiac and statistical experts from across the country, could provide an conclusive answer to claims from the surgeons that acceptable overal death rates had been marred by a "cluster" of unsuccessful operations.

The inquiry panel will hear statistical analysis of results over a 12 year period up until 1995, when the problems at Bristol were exposed.

The study will compare these results with 11 other centres in the UK carrying out similar operations during the same period.

Detailed look

Another study has looked in detail at 80 cases of adult and child heart surgery picked at random from more than 1,800 undertaken over the 12 years.

It may help the inquiry find out whether it was the quality of the surgery that led to any increased death rates, or some other factor such as the quality of intensive care following surgery.

The operation at the centre of the the GMC case was the arterial switch, which is carried out on babies born with a defect which means their heart blood vessels have developed wrongly.

It involves "swapping" the positions of the two arteries connected to the heart.

Death rates in this procedure in the few years leading to 1995 were apparently higher at Bristol, than at other UK centres.

Surgeon James Wisheart, and hospital manager Dr John Roylance were both struck off by the GMC after it decided that the operations should have been stopped earlier as the poor results became obvious.

Another surgeon, Dr Janardan Dhasmana, was banned from operating on children, and was subsequently sacked by the hospital trust.