Death rates in Britain's worst NHS heart units are three times higher than in those of the best, according to figures to be published tomorrow.

The alarming findings from around 30 of Britain's leading heart specialist units will be challenged by the country's top heart surgeons who believe they are misleading.

The gradings of NHS heart units have been produced by a healthcare information business, Dr Foster, publisher of The Hospital Guide. But they have the tacit support of the Department of Health and Alan Milburn, the Secretary of State, who is planning to give patients more choice over the hospitals they can go to for treatment. Mr Milburn this week will also announce that the Government is to provide its own league tables of death rates in Britain's NHS specialist hospital units.

A spokesman for Dr Foster said its league tables are based on raw material produced by the Department of Health and adjusted by experts to take account of factors such as the seriousness of the patient's condition to enable comparisons to be made.

Gradings will be released for coronary unit mortality rates, hospitals meeting national targets for tackling heart disease, neurology and stroke unit mortality rates, the volume of orthopaedic operations including spinal surgery and hip replacements, and general surgery rates.

The gradings are certain to cause alarm among the medical profession. But the Society of Thoracic Surgeons (STS) is ready to challenge the findings by publishing its own data.

"We think patients have a right to know what the results are in their hospitals but it has got to be absolutely correct," said a spokesman. "We don't want to see units penalised because their mortality is high."

Dr Foster's gradings were produced by a research team under Sir Brian Jarman, a leading health expert, who claims they have been adjusted to account for differences between units. The STS will claim that the data has not been properly validated and could be misleading.

The heart unit at Bristol Royal Infirmary is expected by the STS to be among the hospitals to be given a high grading. The unit was at the centre of an earlier scandal over baby deaths, which led to a public inquiry. The inquiry report by Professor Ian Kennedy last year led to the heart surgeon at Bristol, James Wisheart, being struck off.

"I think Bristol is a very good unit. It has made huge strides since the inquiry and I would be extremely surprised if Bristol was not near the very top of the gradings," said the STS spokesman.