Lord Lucan's daughters said a "boyfriend" was staying at their home at the time the family's nanny was murdered, according to newly disclosed police statements.
Lady Sarah Gibbs, his sister, told police at the time that the 7th Earl of Lucan's daughters said another man was at the home, a BBC documentary will claim tonight.
The earl disappeared after the death of Sandra Rivett in 1974 and an inquest found he had murdered her. But he later claimed that he saw a man fighting with Lady Lucan, his estranged wife, on the night of the murder.
In the statement, made in November 1974, Lady Sarah said Lady Camilla Bingham, then a four-year-old, told her an unidentified man sometimes slept in "nanny's room and nanny sleeps with us".
She said: "We were talking about home, that is 46 Lower Belgrave Street, and Camilla said the boyfriend always stays upstairs while we have lunch until we ring the buzzer.
"I said to her 'What's his name?' She said: 'I don't know, he hasn't told me his name.' I said: 'Where does he live?' She said: 'He lives in the house with us.'"
Asked whether the man was the nanny's boyfriend or Lady Lucan's, Lady Sarah, who died in 2001, responded: "I wouldn't know, I'm assuming she meant the nanny because Frances [her 10-year-old sister] referred to the boyfriend when I told her that the nanny was dead."
The evidence was disclosed after the BBC was handed notebooks and tapes by the daughter of the late David Gerring, a detective chief inspector who worked on the case.
Hugh Bingham, the earl's brother, told the BBC he was irritated the witness statement was not heard at the inquest and called for the verdict to be set aside.
"I'm encouraged by the idea that there is a fresh source of evidence and that I feel wants to be given a fair hearing," he said.
"Let us hope that now with the way in which evidence seems to be gathering, there is a chance that maybe the inquest result could be set aside.
"If that happened then the warrant of arrest would fall away and my brother's situation would be restored to the normal situation of a man innocent until proven guilty."
Retired Det Sgt Graham Forsyth, who witnessed the statement, said he did not think the man was traced because it was thought Lord Lucan was the murderer "as was subsequently found at the inquest".
But a spokesman for Lord Lucan's son, George Bingham, criticised the BBC for making the documentary.
"Camilla has no recollection of this whatsoever," he said. "Why would she? She was four years of age.
"We asked to view the programme and the BBC refused which I found quite extraordinary.
"It seems to be a patchwork of theories and suppositions – there is no hard fact in this and the feeding frenzy will continue. If there is actual evidence there it should be given to the police but there isn't."
Inside Out will be broadcast on BBC One in the South East and London tonight at 7.30pm.