Scotland Yard launched an investigation last night into allegations that Tory politicians were members of a paedophile ring which abused children in care in the 1980s.
Operation Fernbridge will examine alleged historical sexual abuse of children at Elm Guest House, in Rocks Lane, a suburban street in Barnes, south-west London.
Residents of a nearby care home run by Richmond Council claim that they were sexually assaulted at the property by a network of prominent individuals, including Tory MPs, who used their connections to escape justice.
The allegations are one of several lines of inquiry being considered by specialist detectives at the country's biggest force as a result of information supplied to them by the Labour MP Tom Watson.
In a surprise intervention in the Commons in October last year, Mr Watson - who alleged widespread phone hacking at the News of the World before police began a new inquiry - urged Scotland Yard to reopen the evidence file on Peter Righton, a former child care consultant who was convicted of importing illegal homosexual pornography in 1992. Saying that the file contained "clear intelligence of a widespread paedophile ring," Mr Watson said at Prime Minister's Questions: "One of its members boasts of a link to a senior aide of a former Prime Minister, who says he could smuggle indecent images of children from abroad.
"The leads were not followed up, but if the files still exist, I want to ensure that the Metropolitan Police secure the evidence, re-examine it, and investigate clear intelligence suggesting a powerful paedophile network linked to Parliament and No 10."
In secret, five officers on the Met's Child Abuse Investigation team began a scoping exercise, Operation Fairbank, to see if the MP's claims merited further inquiry. Working in secret in Earl's Court, they interviewed several adults who had contacted Mr Watson with information until December, when police confirmed the existence of the inquiry.
Last night, Scotland Yard confirmed the scoping exercise had reached sufficient seriousness that it passed "the threshold for a criminal investigation" and announced the creation of Operation Fernbridge.
It is being run by the Specialist Crimes and Operations Directorate, which is known for tackling complex crime and which investigated phone hacking and other illicit newsgathering tactics.
In a statement, Scotland Yard said: "The Metropolitan Police Service have today, Thursday 17 January, launched an investigation, Operation Fernbridge, into historic allegations of child abuse in the early 1980s at the Elm Guest House, Rocks Lane, Barnes, London. The investigation will be led by the Child Abuse Investigation Command.
"The allegations under Operation Fernbridge were initially assessed under Operation Fairbank which was information passed to police by MP Tom Watson. Operation Fernbridge reached the threshold for a criminal investigation. We will not be providing a running commentary on this inquiry."
The police stressed that the allegations were historical and did not relate to the current occupants of the property, which has been under new ownership for some time.
A spokeswoman for the Met confirmed that Operation Fairbank would remain in existence and concentrate on other lines of inquiry provided by Mr Watson, who has been in regular contact with detectives. Those other lines of inquiry also involve politicians.
Whisteblowers with backgrounds in child protection contacted the MP in October as a result of the publicity surrounding the exposure of the BBC presenter Jimmy Savile as a paedophile. He took time to research and assess their claims before deciding to speak out publicly in the Commons. In the ensuing weeks, the former Tory party treasurer Lord McAlpine was wrongly identified by social media users as having abused children at care homes in North Wales, leading to him winning large libel payouts.
Conservative politicians and newspaper columnists accused Mr Watson, who had not named anyone suspected of abuse, of irresponsibly sparking a witch-hunt. Last night Mr Watson said: "This new criminal investigation is welcome news. I am confident that the Operation Fairbank team are committed to a thorough investigation that leaves no stone unturned. As they get nearer the truth it is vital they are given the time and space to conduct the investigation.
"I urge any witnesses or victims to come forward if they think they can help with the inquiry. No matter what has happened in the past, now is the time for your voice to be heard."
The police have asked anyone with information to call their hotline on 020 7161 0500.