Five doctors are under investigation by the General Medical Council for failing to report a colleague who had sexually abused young men during consultations over a period of 12 years. The doctors were partners at the same practice with GP, Peter Green, who was jailed for eight years in July 2000 for indecently assaulting five male patients.

A report on the case published on Thursday by the Commission for Health Improvement (CHI), the government NHS watchdog, castigated an "NHS culture that did not listen to patients" and a complaints system that failed to identify professional misconduct for allowing Green to avoid detection for so long.

Between 1985 and 1997, concerns about Green's behaviour were raised on 23 occasions with different individuals and organisations but no one pieced the jigsaw together, the CHI said. There were three separate police investigations, the first two in 1985 and 1993, but neither of these resulted in action.

The report criticises the NHS complaints system for failing to log and track complaints, including anonymous ones, to spot patterns, and the lack of accountability within primary care. As a result, Green was able to continue with his activities, causing trauma and distress to an unknown number of patients, for years after he should have been stopped. "Patients were left powerless," the report says.

Although Green was convicted on nine charges of assaulting five men a further 21 charges were left on file. He is appealing against his convictions. At his trial last year the jury was told that Green picked his victims like a burglar picks a house, chosen because they were vulnerable and unlikely to complain about their ordeal.

Green had established a "back research project" in which he checked the alignment of young sportsmen's backs as a cover for his activities. But the male patients, all in their teens or early 20s, were told they had a low sperm count and might not be able to have children unless they took part.

They were required to fantasise about sexual encounters, arouse themselves and in some cases masturbate in front of him.

One patient, named as Mr "I", who was 15 at the time, told the court he had gone to the surgery with his girlfriend to ask about a lump on his testicles. Green asked the girl to arouse her boyfriend who was than asked to give a sperm sample. He was told he had a low sperm count and further tests would be needed.

Despite the regular flow of complaints about Green's activities, decisive action to stop him was taken only in June 1997 after a health visitor, who had heard complaints from two patients, persisted in raising the alarm. She was commended for her determination in the report.

The Pinfold Gate Medical Centre in Loughborough, Leicestershire, where Green worked, is regarded as among the leaders in primary care with good organisation and teaching and high levels of patient satisfaction. However, as concerns started to emerge about Green's practice, "there remained an acceptance of his word against that of others," the CHI report says. "This was in spite of indications that the situation as presented by Peter Green was not supported by events, as they became known."

The practice partners even imposed a "sexual conduct protocol" in response to concerns about Green's practice, which included requiring patients to be chaperoned for intimate examinations and audit of patient notes, but then failed to ensure he was following it. Staff at the practice described him as "imposing and authoritative" and were reluctant to challenge him.

Dr Peter Homa, the chief executive of the Commission for Health Improvement, said: "Peter Green's patients were failed not only by him but by a system which allowed a credible person to do incredible things to patients to whom he owed a duty of care. That this continued despite concerns raised by patients and NHS staff over many years is simply unacceptable."

Asked if the abuse carried out by Dr Green could happen again, Liz Fradd, CHI's director of nursing and the chief author of the report, said: "Nobody can regulate for the criminal or the malicious individual. I don't think you can ever be 100 per cent sure that it could not be happening now or could not happen again. But we believe that if the recommendations are enacted it will minimise the chances of it happening again."

Lord Hunt, the health minister, said many of the reports recommendations, such as reform of the NHS complaints procedure due to be implemented by April 2003, were already in train. Tougher lines of accountability for GPs were also being introduced.

"The vast majority of GPs in this country work extremely hard and effectively for their patients...What is really important is that a culture change is taking place. There is a greater openness and acceptance amongst health professionals and managers about their accountability to patients and the public.

"In the new NHS patients and professionals must be confident that they are able to voice their concerns, that they will be listened to and dealt with fairly and effectively," Lord Hunt said.