Mr Mehrotra's son Vishal was abducted as he walked home to Putney after watching the Prince of Wales and Diana Spencer ride to their wedding in a carriage on July 29 1981.
He had gone ahead of other family members for the last few hundred yards. He was last seen less than a mile from the guesthouse.
Mr Mehrotra claims he received an anonymous call from a male prostitute in the months following. A man he guessed to be in his 20s told him Vishal may have been abducted by "highly placed" paedophiles operating from the Elm Guest House, Mr Mehrotra said.
He told The Telegraph: "I was contacted by a young man who seemed to be in his 20s. He told me he believed Vishal may have been taken by paedophiles in the Elm Guest House near Barnes Common.
"He said there were very highly placed people there. He talked about judges and politicians who were abusing little boys." Mr Mehrotra, a solicitor who was a JP at Wimbledon magistrates' court until retiring in 2006, claims the man said he had already informed police about activities at the guesthouse, but had received no response.
He added: "I recorded the whole 15-minute conversation and took it to police. But instead of investigating it, they just pooh-poohed it and I never heard anything about the tape again. The whole thing went cold.
"At that time I trusted the police. But when nothing happened, I became confused and concerned.
"Now it is clear to me that there has been a huge cover up. There is no doubt in my mind."
In February 1982, part of Vishal's skeleton was found in woodland in West Sussex. There was no trace of his legs, pelvis or lower spine, nor of his outer clothes, his sleeveless vest or his Superman underpants.
At the inquest into his death, the West Sussex coroner Mark Calvert Lee recorded an open verdict but said "foul play" was likely.
Police said 20,000 people had been interviewed, half of them in nearby Putney, and 6,000 properties checked.
Mr Mehrotra, now 69 and living in West Molesey near Hampton Court, said he had "hardly been contacted" by police in the intervening years.
He said he had not been spoken to in recent months despite the alleged witness reporting the murder of three boys at the time Vishal vanished.
Mr Mehrotra said: "This guesthouse was right next to where Vishal disappeared. There were predatory people there who were taking young boys and abusing them.
"It seems to me that it all adds up, so I can't understand why the police have again failed to get in contact with me. I think the revelations of Savile and others in recent months have opened up a Pandora's box. Hopefully everything will all come out soon."
In June 1982, four months after Vishal's remains were found, police raided the Elm Guest House.
Dozens of men were questioned, reportedly including at least 30 who were prominent in public life and business. It was widely reported at the time that the raids were linked to Vishal's disappearance. The Times reported that the investigation had included the disappearance of another boy, Martin Allen, 15, missing since Guy Fawkes Night, 1979, whose body has never been discovered. The son of the chauffeur to the Australian High Commissioner, he was last seen waving goodbye to a school friend at King's Cross Underground station.
Police at the time dismissed the reports as "nonsense". Soon afterwards, lawyers acting on behalf of the guesthouse threatened newspapers with legal action if they continued reporting on its alleged activities.
Martin's brother said on Tuesday that police should reopen the investigation into the teenager's disappearance. Kevin Allen, 51, said he had always suspected a cover-up after police told him all the case files had been lost in a freak flood.
He said: "I think it's a new lead. Anything to ensure these people don't get away with it. I think there are powerful forces involved in this. Years ago I was warned by a policeman that if I looked too deep into this then I might get hurt. I've never forgotten that.
"We have barely heard anything for 20 years, but there are other missing cases where the police barely stopped looking.
"My dad died never knowing what happened to Martin. We would love to have an answer for my mother before she passes away."
In May 1983, as police wound up the inquiry into Vishal Mehrotra's death, Carole and Harry Kasir, the owners of the Elm Guest House were fined £1,000 each and given suspended nine-month sentences at the Old Bailey for "running a disorderly house". They were found not guilty of living off immoral earnings and having obscene films.
Five years later Carole told child protection officers that children from the council-run Grafton Close Children's Home had been supplied to the brothel. She provided names of people who had frequented the guesthouse.
The Liberal MP Cyril Smith, now dead, has been widely alleged to have abused children from Grafton Close at The Elm.
At an inquest into her death in 1990, members of The National Association of Young People in Care said that Kasir had lived in fear of her life since the hotel was exposed. Christopher Fay said: "The reasons for her death are all tied up in this child pornography ring at the hotel.
"She was hounded and harassed by police and security services. She knew all the top people who had been involved in the ring at the hotel."
Scotland Yard launched Operation Fairbank two years ago to look into suggestions that high profile political figures had been involved.
Officers have set up a new strand of the inquiry, Operation Midland, after being passed information about the three alleged murders.
The allegations emerged when a man in his 40s came forward claiming to have been one of around 15 boys who were abused by a powerful paedophile network 30 years ago.
Some of the abuse allegedly took place at flats in the Dolphin Square development in Pimlico, where a number of politicians have had London homes.
According to the man, a 12-year-old boy was strangled by a Conservative MP at a town house in front of other victims.
On another occasion, a boy of around 10 was deliberately run down and killed by a car being driven by one of his abusers, the man claimed.
The Attorney General on Tuesday said he would back an investigation into the allegations if there was evidence to support the claims.
Jeremy Wright, speaking in the Commons, said: "My view is that the Crown Prosecution Service should pursue cases where the evidence exists to wherever the evidence leads, and that is regardless of the position held by the person being investigated.
"And if evidence is brought to light to justify such an investigation, I would expect it to be carried out."
A spokesman for the Metropolitan Police said the force would not comment on an ongoing investigation.
Additional reporting by Ben Parsons
