The credibility of government claims to be cutting NHS waiting times is put in doubt today by a report from the audit commission showing widespread error in hospitals' official waiting list records.

In a spot check of 41 NHS trusts, the commission found three had fiddled the figures and 19 made mistakes in information that will be used to assess whether ministers have met their targets for reducing waiting times.

One hospital's figures were declared impossible to assess because its information technology was so inadequate. And 15 were found to have system weaknesses that increased the risk of reporting errors. That left only three NHS trusts with a clean bill of health. The gravity of the position was compounded when the commission disclosed that it had left two further NHS trusts out of its spot check after strategic health authorities began investigating allegations of waiting list manipulation.

James Strachan, the commission chairman, said: "This report shows widespread inaccuracy in waiting list figures - some deliberate but much due to ineffective management or inadequate systems."

The report said most of the mistakes probably did not cause significant delay to patients' treatment, but some were clearly unreasonable. Sir Nigel Crisp, the NHS chief executive, said deliberate misreporting of waiting list data was "reprehensible and inexcusable". Where it occurred there would be "serious consequences".

To find three trusts were deliberately manipulating figures was a "very disappointing result and is very sad for the NHS. We cannot get 100% accuracy, but we can do a lot better than this," he said.

The 41 trusts in the survey were selected after auditors spotted discrepancies that suggested there might be some irregularity. So it was wrong to regard them as representative.

The government's current target is to reduce inpatient waiting times to a maximum of 12 months and outpatient waits to a maximum of 26 weeks. Hospitals in the survey tended to be more accurate with this data than with other material, he said.

Sir Nigel ordered the spot checks after the national audit office discovered in December 2001 that 6,000 patients waited longer than they should for treatment because lists had been fiddled at nine NHS trusts.

The commission said the manipulation it had discovered included one hospital with internal records showing a waiting list of 5,800 patients, but pretending that it was 3,800 whenever figures were reported externally.

Another hospital removed patients from the waiting list by pretending they had been given a "planned admission date".

The patients were not told and were returned to the waiting list after an official return had been made to the Department of Health. Sir Nigel said hospitals criticised by the commission should report to him by the end of the month on how they were solving the problem. In addition, all hospitals should report by the end of June on how they were addressing the issues.

Who massaged the figures
And who won a clean bill of health

How the audit commission ranked 41 NHS trusts in a spot check of waiting list records:

Deliberate misreporting: three

· East and North Hertfordshire

· South Manchester University hospitals

· Scarborough and North East Yorkshire Healthcare

No assessment possible (trust could not provide information): one

· Bromley hospitals

Evidence of reporting errors: 19

· Bedford hospital

· Berkshire Healthcare

· Central Manchester & Manchester Children's University hospitals

· Cambridgeshire and Peterborough Mental Health Partnership

· Countess of Chester hospital

· Dartford & Gravesham

· Hammersmith hospitals

· Local Health Partnerships (mental health)

· Luton & Dunstable hospital

· Moorfields Eye hospital

· Northgate and Prudhoe

· Portsmouth hospitals

· Royal National hospital for rheumatic diseases

· Royal Orthopaedic hospital

· United Bristol Healthcare

· Walton Centre for Neurology & Neurosurgery

· Worcestershire Acute hospitals

· Wrightington, Wigan & Leigh

· York Health Services

System weaknesses increasing risk of error: 15

· Epsom & St Helier hospitals

· James Paget Healthcare

· Leicestershire Partnership

· Nuffield Orthopaedic

· Royal Liverpool & Broadgreen University hospitals

· South Buckinghamshire

· South of Tyne & Wearside Mental Health

· South Tyneside Healthcare

· Surrey Oaklands

· United Lincolnshire

· University hospitals Coventry & Warwickshire

· West Hertfordshire hospitals

· West Suffolk hospitals

· Winchester & Eastleigh Healthcare

· Wirral hospital

Clean bill of health: three

· Nottingham City hospital

· Royal Devon & Exeter Healthcare

· Salisbury Healthcare