The first-ever detailed study of "adverse incidents" in NHS hospitals has found thousands in just a handful of trusts over a six month period.

But a storm has erupted over claims that Health Secretary Alan Milburn tried to prevent the release of the figures to avoid creating alarm among patients.

This has been denied by sources at the Department of Health, who said it would be "irresponsible" to publish preliminary, and possibly unreliable data in an official document.

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The figures were collected by the newly-created National Patient Safety Agency. It found more than 24,500 "adverse incidents" in the 28 participating trusts over a six month period.

However, there are fears that this figure does not give an accurate representation of the true level of errors and accidents at NHS hospitals.

There is nothing to hide and we are committed to publish audited figures when they are available

Department of Health spokesman

This is because some of the hospitals in the study reported even relatively trivial problems, such as a bandage being wrongly applied, while other hospitals may have been under-reporting their incidents.

Professor Sir Liam Donaldson, Chief Medical Officer for England, told the BBC: "We were worried about publishing data that had such a high level of inaccuracy."

But a Department of Health spokesman said that the figures were broadly in line with other countries.

"One of the major question marks about the quality of the data is that because the pilot scheme is in its early stages the number of reported incidents is less than international studies would suggest for comparable health care systems."

Fresh idea

The research is unique because it is the first time that NHS staff have been openly encouraged to report "adverse incidents" - situations in which patients have been placed at risk.

It is part of government drive to reduce a "blame culture" among doctors and other health professionals.

Ministers want to set up an "early warning system" which could spot common mistakes and hopefully work to reduce them.

A report published in June 2000 suggested that there might be at least 850,000 "adverse incidents" in the NHS every year, hundreds causing severe health problems or the death of the patient.

One in 10 patients entering hospital may be affected by one, say experts.

If patients are being put at risk there needs to be a full public and transparent debate about putting things right

Dr Liam Fox, Shadow Health Secretary

More than 50% of the annual litigation bill for the NHS comes as the result of mistakes during childbirth which result in brain-damaged babies.

The NPSA pilot study reportedly found more than 300 incidents involving mistakes during childbirth.

Whether the 24,500 total is sufficiently robust to be extrapolated across the NHS is disputed, but if the 28 trusts were representative of the NHS as a whole, then it could mean more than a million accidents and errors every year.

Susan Williams, NPSA joint chief executive, said the data was not robust, and further work was needed.

"One of the major issues the pilot has revealed is the difficulty in providing the accurate information required by the NPSA.

"We are committed to publishing full figures as soon as the further analysis is complete."

Mike Stone, from the Patients' Association, said that the figures should be seen as a "starting point".

"Figures like these are very very important - they give the public a snapshot of what is going on."

The Conservative Shadow Secretary of State for Health, Dr Liam Fox, said: "The public have a right to know what is happening in their own health service.

"If patients are being put at risk there needs to be a full public and transparent debate about putting things right.

"Covering up important data makes people more suspicious."

The World Medical Association is calling for extra effort to be made to cut errors and accident.

Dr Delon Human, secretary general of the WMA, said: 'It is now recognised that thousands of patients die every year across the world, not because of disease but because of health system failure."