The Metropolitan Police was told sex offence units were overstretched months before the arrest and release of taxi rapist John Worboys, it has emerged.
Worboys 51, of Rotherhithe, south London, was convicted in March of attacking 12 women.
The Independent Police Complaints Commission is investigating why he was released after an arrest in July 2007.
A report said specialist sex offence units, known as Sapphire teams, were understaffed and overstretched.
A report by Government inspectors, which was seen by Met chiefs nine months before the original arrest and release of Worboys, found that Plumstead CID, in south east London, needed urgent reform.
Ch Supt Nicola Grevatt, of HM Inspectorate of Constabulary, said the Greenwich borough only had six officers with specialist training to deal with sex offences.
Detectives were investigating up to 20 rape investigations each, the report added.
The effect had been that rape victims were losing contact with the police or withdrawing their support.
Ms Grevatt also said senior officers were responsible for poor communication, in-fighting and an atmosphere of political correctness and paranoia.
Worboys was held at a police station in Plumstead, south-east London, but released on bail.
In February last year he was arrested for a second time, which led to the charges on which he was convicted.