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, an official report has said.

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."

Rape victims were pressured to withdraw their allegations by a specialist Metropolitan police unit as officers tried to gerrymander their performance statistics, the report found.

The Independent Police Complaints Commission (IPCC) said the Sapphire unit at Southwark had failed victims. Women reporting rapes and sexual assaults were encouraged to withdraw their allegations, it said.

The watchdog said its investigation followed cases in 2008, when the Southwark Sapphire unit was performing poorly and under pressure to improve.

The report found women were questioned repeatedly by a detective about whether they had consented to sex. They were encouraged to withdraw their complaints, which boosted the unit's sanctioned detection rate.

The rape allegations were not recorded by police.

The IPCC deputy chair, Deborah Glass, said the unit's approach was "wholly inappropriate" and "deeply disturbing".

The IPCC said the unit had improved since coming under centralised command in 2009.

The Metropolitan police said in a statement: "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."

The statement said: "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. There is also more focused engagement with external scrutiny bodies ensuring the MPS learns lessons and continues to improve performance and public confidence."