According to an exclusive story in the Daily Mirror, Neil Berriman said Lucan was now living as a Buddhist in a shared house.
He said: "I believe I have tracked down the man, Lord Lucan, who murdered my mother.”
Lord Lucan vanished after the murder of nanny Sandra Rivett at the family’s exclusive mews home in Belgravia, Central London, on November 7, 1974.
Police believe he attacked Miss Rivett with a taped-up piece of lead piping after mistaking her for his wife, Veronica.
He and his wife had a volatile marriage and were going through a bitter custody battle over their three children.
At the time there were suggestions Lucan had committed suicide by throwing himself off a cross-Channel ferry - but no body was ever found.
Lord Lucan had many powerful, wealthy friends who it is speculated may have helped him escape a prison sentence.
There have been sightings over the years in South Africa, the Maldives and India.
But Mr Berriman - who had been adopted shortly after birth - is confident Lucan is now living in Australia having received a detailed tip-off letter.
He said the man he believes is Lucan - who would now be 85 - needs a part-time carer, and often sits on a veranda listening to trains.
The friends confirmed to the Daily Mirror that an elderly Englishman who looks like Lucan lives at the house.
Mr Berriman said: “The people he lives with know he has a mystery past and what he tells them does not add up. They have had their suspicions for many years.
“Lucan is a deceitful conman and he is the man who murdered my mother.
“There is absolutely no doubt in my mind he escaped that night, with the help of friends who helped him get across the Channel and get a new passport, and incredibly he is still alive.
“From my own inquiries he’s had at least six different identities."
Scotland Yard said it would not comment on the case.