A maverick Scotland Yard rape squad allowed a rape suspect to go unquestioned and free to later kill his two children, it has emerged.
A woman who alleged Jean Say, 62, had raped her was persuaded to retract her complaint by untrained officers under pressure to hit crime targets, said the Independent Police Complaints Commission watchdog.
Say later slashed the throats of his two children, aged 10 and eight, when they went to stay with him for a weekend two years ago.
He was jailed at the Old Bailey for a minimum of 30 years.
The IPCC report has condemned the squad, a specialist Sapphire unit investigating rape and serious sexual crimes in Southwark, South London.
The rape allegation against Say was dismissed by a detective sergeant in the Sapphire unit who said the circumstances did not constitute rape because the woman "consented".
The IPCC said: "The findings of our investigation into the rape ... were also deeply disturbing. The victim was failed by the people from whom she sought help."
It added: "There is no doubt from the evidence that the woman made an allegation of rape at Walworth police station which should have been believed and thoroughly investigated."
Deputy chairwoman of the IPCC Deborah Glass said: "There's no doubt this was an incredibly serious, shocking incident. We know with all the cases that we've dealt with that the consequences of not dealing with allegations of rape can be extremely serious. This is yet another tragic illustration of that."
It has also emerged that three senior officers recommended by the IPCC for gross misconduct disciplinary hearings around failures in another case escaped serious sanctions. Two were promoted.
The IPCC has investigated the Met's sexual crime squads nine times in recent years, five probes involved the Southwark squad. Nineteen officers have been disciplined, three dismissed and one was jailed.
Ms Glass added: "Given the number of cases where the (Metropolitan Police Service's) response to victims has failed, either through individual officers' criminality or neglect or more systematic problems of training, priorities and resources, the response that 'lessons have been learned' begins to ring hollow."