A British judge declared the infamous Lord Lucan officially dead on Wednesday, four decades after he disappeared following the murder of the family's nanny in a lurid tale that has gripped Britain.

"The court must make the declaration that is sought on this case," Judge Sarah Asplin told a court in London following an application by Lucan's son George Bingham, who officially becomes the 8th Earl of Lucan.

Bingham, 48, launched a High Court bid to obtain a death certificate for his father last year, bringing to an end one chapter in a story full of unanswered questions that revolves around London high society of the 1970s and its gambling underworld.

"I am very happy with the judgement of the court in this matter. It has been a very long time coming," Bingham told reporters after the hearing, flanked by his new wife Anne-Sofie Foghsgaard, the daughter of a wealthy Danish industrialist.

"I got married this year. I've started a new branch in my career. It was a nice moment to say farewell to a very distant past and to move on at a very personal level," he said.

"Our family has no idea how our own father met his own end. Whether he did so at his own hand or at the hand of others.

"It's a mystery and it may well remain that way forever."

Intrigue has shrouded the whereabouts of Richard John Bingham, the 7th Earl of Lucan, since he vanished at the age of 39 after his children's nanny, Sandra Rivett, was bludgeoned to death in 1974 at the London home of Lucan's estranged wife.

An inquest into the murder raised the theory that Lucan may have mistaken the nanny for Lady Lucan, who said she fought with the attacker in the dark after she heard noises in the basement.

The case sparked worldwide interest after his blood-soaked car was found abandoned near the coast, and since then there have been dozens of supposed sightings of him from southern Africa to New Zealand.

Bizarre conspiracy theories abound, with one of Lucan's old gambling friends claiming last week that he committed suicide and was then fed to a tiger in a zoo to avoid leaving proof of death and handing his estate over to estranged wife Veronica.

Police issued a warrant for his arrest a few days after the murder.

Bingham began the legal process last October by posting a public notice in a local newspaper. He has said the death certificate would "bring closure".

Life-changing discovery  

Neil Berriman, the murdered nanny's biological son, withdrew an earlier objection to the application, but told court on Wednesday that he had seen an internal police document suggesting Lucan "was possibly alive from 2002 onwards".

He rejected the commonly-held explanation that Lucan drowned at sea, and accused the Metropolitan Police of corruption, calling the force "a firm within a firm".

"Maybe the police know more than they let on," he said in a statement outside court, in front of a press scrum.

The 49-year-old builder said Lucan "was guilty of something" but that he wasn't "100% sure" that he actually killed his mother.

"I think he escaped," he said, adding that it was possible he fled to Africa, although was now probably dead.

Berriman earlier told AFP that Bingham was "selfish" to say he wanted closure.

"Closure for us is when we can actually get closure for Sandra," he added.

Berriman, who was put up for adoption at birth, only found out he was Rivett's son when his adoptive mother died and left him a brown envelope, which he opened eight years ago.

"It was full of all sorts of things, birthday cards and cuttings of the Lucan case, which was very bizarre," he told AFP.

"I sat there and it gradually registered.

"It was horrendous, things haven't been the same since."