Cyril Smith was able to live a high-profile life as Rochdale MP after the Metropolitan Police shelved an investigation into a alleged Westminster child abuse ring, a former officer has claimed.
Smith, who died in 2010, is one of a group of men that were apparently investigated by the Met in relation to abusing boys aged around 14, an undercover source told BBC Newsnight.
It was claimed Smith was caught engaging in a sex party with teenage boys at a property in Streatham, south London.
He was taken back to a central London police station but later released. Two senior police officers were also reportedly implicated in the sex abuse ring.
The source said that starting in 1981, police were gathering evidence on addresses in the south of the capital, with one focus being a flat in Coronation Buildings, Lambeth - just one mile away from the House of Commons.
But a senior officer at the force brought the probe to an end after Smith and others had been arrested.
The source told the BBC that officers were ordered to hand over all their evidence - including notebooks and video footage - and read the Official Secrets Act.
Simon Danczuk, Labour MP for Rochdale, who brought attention to Smith's double life as a child molester, said Smith had been protected.
"Time and again what we have learned more recently is that a number of police officers investigated Smith up and down the country and those investigations were quashed and officers were told to stop investigating," Danczuk said.
"It is my view that Smith was being protected and being protected by some fairly powerful people. It is my view he was protected because he knew of other paedophiles in the networks in which he operated and had he been prosecuted then I think those other people would have been named by Smith and that's why they ensured they would never be put before the courts."
The Independent Police Complaints Commission is investigating 14 allegations of corruption in the Met in relation to child sex offences dating from the 1970s - 2000s.