David Britten, a manager at the Peter Dally Clinic in Central London, was dismissed from the clinic in 2002 by Central and North West London Mental Health Trust as a result of a professional misconduct charge.
But his sexual activities were only discovered following his dismissal, when a number of former patients approached the trust with allegations against him.
Starting in the early 1980s Britten had sexual relationships with women while they were in his care, the report said.
The investigation by NHS London concludes that Britten's behaviour went undetected because of poor management.
It also criticises the Nursing and Midwifery Council for not suspending Britten from the nursing register until 2004, despite allegations of sexual abuse against him in 2002.
The Peter Dally clinic closed in 2001 after Britten's activities were discovered. A year later a new eating disorder service with new staff was opened.
A statement from the NMC said: 'The Nursing and Midwifery Council (NMC) apologises to all victims involved in the David Britten case for the length of time it took to issue an Interim Suspension (IS) order. As a registered nurse, Britten's behaviour was wholly unacceptable and demonstrated a serious departure from the standards of practise set out in the NMC Code of conduct'.
Alison McKenna, chair of the investigation, said: 'The effect of David Britten's abuse of these vulnerable women cannot be overestimated. David Britten was a specialist in eating disorders and would have known that affected individuals can be very compliant and eager to please. He deliberately targeted vulnerable patients grooming them for his own sexual gratification'.