One woman died and 10 others had their diagnosis of breast cancer delayed for up to 21 months after staff at one of the biggest breast-screening services in the country bungled the recall of patients.

Confusion reigned at the West of London Breast Screening Service, based at the Hammersmith Hospital, where records were poorly marked, national guidelines were ignored and there was no effective leadership, an investigation by the Commission for Health Improvement (CHI) found.

The report by the NHS watchdog yesterday says an "unacceptable and avoidable failure" led to delays that harmed patients. The records of 100,000 women who had 174,000 screening tests since 1993 were checked and 123 were wrongly given the all-clear.

The failure is the third to blight the NHS breast-screening programme. A similar incident bungling the recall of patients happened at the south Birmingham breast-screening service in 1994. As a result, guidance was issued to all 90 units in the country in 1995 but it was not mandatory.

In 1997, new measures were imposed to improve the quality of breast screening after failures at the Royal Devon and Exeter hospital.

Mistakes had been made at the West of London screening service before but nothing had been done to stop them recurring. The problem stemmed from the confusing way in which records were marked, "RR" for women needing routine recall in three years and "recall-recall" for those who needed to return immediately for further checks.

Labour shortages meant there was a heavy reliance on temporary staff who were not familiar with the difference between the two marks.

Peter Homa, chief executive of CHI, said: "A series of basic errors led to 11 women being harmed by the NHS. This is unacceptable. National guidance was neglected, despite attention being drawn to it and despite files having been mixed up in the past. We need to ensure lessons are learnt across the entire breast-screening service so it does not happen again."

He added: "There is no single individual responsible for this service failure. A number of individuals share a degree of responsibility. It was, in essence, a systems failure."

Dr Linda Patterson, the medical director of CHI, said it was impossible to say whether the woman who died, whose diagnosis had been delayed by 15 months, had done so as a result of the failure. But, she added: "We know early diagnosis and treatment improves survival."

Ironically, the West of London service had a good record of detecting cancers, spotting six per 1,000 women screened for the first time compared with a national average of 3.6 per 1,000. But a simple administrative bungle led to women being sent the wrong results.

The failure was spotted in October 2000 after a woman given the all-clear moved out of the area and discovered from another screening service that she had been sent the wrong result. Managers of the West of London service at first failed to recognise it as a serious incident and it was not reported to the chief executive of the trust until mid-November. When the internal review found a delay in diagnosis of breast cancer for 11 women, CHI was called in to investigate in April 2001.

Delyth Morgan, chief executive of Breakthrough Breast Cancer said: "We need to see clear evidence of improvements and confidence restored in a service that is a vital tool in the fight against breast cancer."