Experts are unable to say how many of the 27,000 medical errors or mistakes reported by a handful of NHS trusts in a pilot study seriously threatened patients.

Officials from the newly-created National Patient Safety Agency said problems with the way the information was collected mean most of the reports cannot be categorised accurately.

The agency said it was now working to overhaul the way reports of "adverse incidents" are collected before the scheme is rolled out across the NHS next year.

Outcome of incidents

62% Not specified 19% Minor 17% None 2% Catastrophic

The announcement came as the agency issued a safety alert on the use of potassium chloride in NHS hospitals and follows accusations on Monday that ministers had tried to stop details of the study results becoming public.

The Department of Health denied accusations of a cover-up saying it would be "irresponsible" to publish preliminary and possibly unreliable data in an official document.

The NPSA received more than 27,000 confidential reports from staff and patients in 28 trusts of incidents where problems occurred.

Technical problems

However, in half of the hospitals computer problems meant all of the information could not be transferred to the agency's main database.

In other instances, staff found the reporting forms too complicated and failed to give all the necessary information.

In all, 62% of the reports could not be classified.

Susan Williams, joint chief executive of the NPSA, told the BBC: "The trusts really struggled with this. Over half of our reports did not have the severity coded onto the information. I think it does present us with quite a lot of problems."

The trusts really struggled with this

Susan Williams, NPSA

She added: "The system we were trying to introduce isn't going to work for all 720 trusts and that is why we are going to be developing over the next three months on an e-based form which is much more straight forward and simpler to use so there will be a standard NPSA form across the NHS."

Positive findings

Professor Sir Liam Donaldson, chief medical officer for England, said the some of the study findings had been positive.

He told the BBC: "What the pilot study has told us is staff are willing to report their errors if they think the information that they give might help a future patient and that is very very good news.

"The less good news is we had a lot of technical problems. Some of the staff found the form too complicated.

"We have got to go back to the drawing board on some of those problems. We are looking at ways of solving that before it goes nationwide next year."

The NPSA study is unique because it is the first time NHS staff have been openly encouraged to report all mistakes and so-called "near misses" to the NPSA.

Eventually all hospitals and GP surgeries will be required to do so.

It is part of a government drive to reduce the "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.

Lessons learnt

Mike Stone, director of the Patients Association, said the figures needed to be put in context.

But he added: "Lessons must be learnt. We are talking about people's lives at the end of the day."

Dr Evan Harris, Liberal Democrat health spokesman, said: "We can only prevent mistakes by changing procedures, so it is essential that people are aware of mistakes when they happen."