A police officer at a sex crime unit encouraged a woman to drop a rape claim against a man who went on to murder his two children, a report has said.
Jean Say killed his son and daughter two years ago.
The Independent Police Complaints Commission (IPCC) said Southwark Sapphire unit in south London "encouraged" victims to withdraw allegations to boost detection rates.
The Metropolitan Police said substantial changes had been made.
The rape allegation against Say was dismissed by a detective sergeant based at Southwark, who said the circumstances did not constitute rape because the woman "consented".
The report by the IPCC 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."
The IPCC said the Southwark Sapphire squad's approach of "failing to believe victims" was "wholly inappropriate". It also said it was "under-performing and over-stretched".
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."
Say, 62, a retired concierge, attacked his daughter Regina, eight, and son Rolls, 10, in their beds at his home in Southwark in 2011, while they were on a weekend access visit.
The Old Bailey heard he killed them to spite his wife who had left him. He was given a 30-year minimum term.
The case sparked a wider investigation into the work of the unit between July 2008 and September 2009, the results of which have now been published.
The IPCC has carried out five previous inquiries into Southwark Sapphire command and it is the police watchdog's ninth investigation into the Met's response to victims of sexual violence.Officers promoted
Home affairs correspondent Danny Shaw said three Metropolitan officers who had been facing possible dismissal over a previous bungled investigation into the serial sex offender Kirk Reid, were still with the force.
Two of them, a superintendent and a detective inspector, have been promoted.
A detective sergeant is facing disciplinary proceedings for alleged gross misconduct in connection with the allegations.
In another case, Det Con 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.
A Met Police spokesman said: "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."