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Police were looking into allegations made about the top Tory politician when he died of cancer last week.

Award-winning investigator Mark Williams-Thomas, the man who exposed Jimmy Savile as a paedophile, confirmed: “Police took the ­allegation very seriously.”

Lord Brittan’s apparent sexual deviancy has been rumoured for decades. But today, for the first time, the Daily Star Sunday can reveal what is actually known to authorities.

We can disclose that Brittan’s alleged victim said he was abused while at primary school.

And we can report sickening testimony from a social worker who describes seeing photos taken in the 1970s of Brittan dressed in kinky outfits with naked boys sitting on his lap.

Brittan is said to have abused children at Elm Guest House, a boarding house in Barnes, west London, run as a brothel in the 1970s and 1980s.

Documents placed online in recent years – but until today unsupported by independent evidence – said he was one of a number of high-profile attendees.

Last night former detective Mr Williams-Thomas spoke out to clarify rumours and ­explain exactly what was being investigated.

He said: “In 2013 the police became aware of information that related to an allegation of sexual assault of a young boy in the 1980s.

“This information related directly to a ­sexual assault alleged to have occurred at the well known and notorious Elm Guest House. It was alleged the offender was Leon Brittan.

“Police took the allegation very seriously and believed it to be credible.”

Mr Williams-Thomas told how Elm Guest House was raided by police in 1982 amid fears children were being abused there.

But he added: “At the time no investigation was done into potential child abuse offences. This was clearly a massive failing by police.

“Following the Savile exposé in 2012, Elm Guest House was investigated again, properly this time – an investigation that remains ­ongoing under the name Operation Fairbank.

“In 2013, officers travelled abroad to see the potential victim. This person is now an adult but was aged ten when the alleged abuse ­occurred.

“The man now lives in the US and, prior to travelling to see him, the British police asked the local sheriff to visit and speak to him.

“The sheriff confirmed the man was ­prepared to make a statement and name Leon Brittan as his alleged abuser.

“Local police added they felt his account was credible. Yet between the time of the local ­police speaking to the alleged victim and the British police flying out to see him and take a statement, he changed his mind.

“I am aware of a specific reason as to why I believe he changed his mind. At this stage I can’t say what this is, other than to say he came under pressure not to give a statement.

“The British police therefore had a ­credible alleged victim and other ­supporting ­evidence from the 1982 raid.

“If he (the alleged victim) had given a signed statement Leon Brittan would have been formally interviewed under caution over the child abuse ­allegation.”

Brittan, who was Home Secretary ­under Margaret Thatcher from 1983 to 1985, was also named by another alleged victim as being among a group of ­VIPs who he saw at paedophile orgies at the Dolphin Square flats near Westminster.

Brittan made no public comment on the ­allegations before his death from ­cancer at the age of 75 on Wednesday. Friends said the politician was ­horrified by the ­accusastions.

Last year he was quizzed by police ­after a woman claimed he raped her in the late 1960s. Police were investigating that case when he died.

Brittan also came ­under fire after ­campaigning MP ­Geoffrey Dickens handed him evidence of a ­Westminster paedophile ring in 1983.

The files went missing and the crimes were never investigated.

Mr Williams-Thomas added: “I have heard the many rumours flying around. This is why I’ve chosen to set out what was known about Leon Brittan and what was being investigated – no rumour, no assumptions, just fact.

“I am doing exactly what Leon ­Brittan’s friends have said – ‘put up or shut up’.

“Ultimately only Lord Brittan knows exactly what happened over his many years – true or false, these are the allegations he faced.”