A man who stabbed his two children to death had avoided a rape inquiry three years earlier because his alleged victim’s claims were written off by investigators on a scandal-hit sex crime unit at Scotland Yard.

Jean Say was named by a woman who reported a rape at a south London police station in 2008, but a detective sergeant said that the woman “consented” to the encounter and no investigation was carried out, according to a report by the Independent Police Complaints Commission (IPCC).

Little more than two years later, Say, then aged 62, killed his son Rolls, 10, and daughter Regina, eight, in their beds at his home on a weekend visit. Say, who was facing eviction after his wife walked out on him, called her and said: “I have killed your children. Come and get the bodies.”

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The police watchdog inquiry into the Jean Say case spiralled and revealed systemic attempts in Southwark to pressure women to retract their statements to classify them as “no-crimes”. The management culture was to hit targets in a unit where resources were stretched and there had been no attempt to remedy the problems, the watchdog said.

Detection rates in the borough had increased from 10 per cent to 31 per cent as a result of the policy.

Victims were closely questioned by a detective constable before talking to a specialist rape investigation 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.

Deborah Glass, the deputy chairwoman of the IPCC, said: “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 investigation was the watchdog’s ninth into the “Sapphire” units – supposed to be specialist sex-crime investigators – with five of those at Southwark. The nine inquiries have resulted in 19 officers being disciplined and three sacked. The former detective constable Ryan Coleman-Farrow, who was based in Southwark, was jailed for 16 months last year for faking police statements to avoid carrying out proper inquiries.

The Jean Say investigation has resulted in a detective sergeant facing a gross misconduct inquiry.

Ms 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.”

The force said that it acknowledged that previous rape inquiries were “below standard” and has made substantial changes to the way it carries out such inquiries. A major restructuring in 2009 followed a series of high-profile failures, including the case of Kirk Reid who was able to operate as a serial sex offender for years in London because of the “shameful” failure to stop him, according to the watchdog.

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The watchdog had called for three officers to face full disciplinary hearings in that case that could have seen them sacked.

But “foot-dragging” meant the hearings never took place and the officers were given only written warnings. Two of the officers have subsequently been promoted, including one to chief superintendent, according to the IPCC. Another senior officer, now a chief superintendent, was given “words of advice”.

In a letter written in 2011 released today by the watchdog, Ms Glass criticised a “litany of delay and prevarication” in bringing the misconduct hearings before they were eventually abandoned. She said that the Met never replied to the letter.

A Scotland Yard spokesman said the head of the force’s disciplinary team had been in regular contact with Ms Glass over the decision: “This matter was subject to detailed consideration and dealt with as appropriate.”

He said supervision of officers’ work had improved. “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.”