Police from a heavily criticised sex crime unit pressured a woman to drop a rape claim against a man who went on to murder his two children, a watchdog said on Tuesday.
Jean Say killed his son and daughter two years ago when they went to stay with him for a weekend.
The earlier rape allegation against him was dismissed by a detective sergeant based in the Sapphire unit at Southwark, south London, who said the circumstances did not constitute rape because the woman "consented".
A report by the Independent Police Complaints Commission said: "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."
The case sparked a wider investigation into the work of the unit between July 2008 and September 2009, the results of which were published today.
It is the fifth into the Southwark Sapphire squad and the ninth into Scotland Yard's handling of sex crimes.
The report found that the unit was "underperforming and overstretched" during the period in question.
Victims were pressured into giving retraction statements, which meant that the alleged crime had not taken place and boosted detection rates.
The report said victims were closely questioned by a detective constable before talking to a specialist officer.
This meant they were questioned repeatedly and went against standard practice that a victim should be believed in the first instance until evidence showed otherwise.
In a foreword to the report, Glass said: "Today's report brings to an end the IPCC's involvement in this sorry chapter of the Sapphire Unit's history.
"The approach of failing to believe victims in the first instance was wholly inappropriate. The pressure to meet targets as a measure of success, rather than focusing on the outcome for the victim, resulted in the police losing sight of what policing is about - protecting the public and deterring and detecting crime.
"The findings of our investigation into the rape reported in November 2008 were also deeply disturbing. The victim was failed by the people from whom she had sought help.
"Since 2009, when the unit came under central command, Sapphire has changed considerably and continues to evolve.
"But given the number of cases where the MPS's response to victims has failed, either through individual officers' criminality or neglect or more systemic problems of training, priorities and resources, the response that 'lessons have been learned' begins to ring hollow."
This is the latest investigation into the handling of sex crimes by Scotland Yard.
The force has previously been criticised for failures over serial sex offenders sports coach Kirk Reid and taxi driver John Worboys.
In another case, Detective Constable Ryan Coleman-Farrow, who was based in Southwark, was jailed for 16 months in October last year for failing to investigate rape and sexual assault claims.
A second officer, based in Islington, north London, is still under criminal investigation.
In total 19 officers from across London have been disciplined, including three who have been sacked.
However, it also emerged today that Scotland Yard has failed to bring gross misconduct proceedings against three officers involved in the Reid case.
This was despite the IPCC recommending that the hearings took place, and a letter sent to the force by Ms Glass expressing her frustration at delays.
In fact, two of the officers involved have since been promoted.
Scotland Yard said it welcomed the findings of the report.
The force released a statement to say: "We have for some time acknowledged that previous investigation of rape and serious sexual assault in the MPS was below standard.
"The activities identified in this report came during that era and highlight specific issues within Southwark which resulted in unacceptable actions by local officers.
"It is as a result of such failings that we have made substantial changes to the investigation of rape and serious sexual assault, both in terms of structure and revised working practices.
"In 2009 Sapphire was moved to Specialist Crime and Operations to increase the focus on victim care and improve investigation standards. This has led to increased supervision and the Met being much better placed to identify any wrongdoing and refer it to the IPCC. The number of prosecutions has increased by 18% and victim care has improved significantly.
"We are not complacent and know there is always more that can be done to improve our service to victims. That is why we continue to work closely with key partners including the CPS, the Havens and charities such as Rape Crisis."