George Bingham, the only son of missing peer Lord Lucan, has been granted a death certificate by a High Court judge in London.
Bingham applied under the Presumption of Death Act, which came into effect in 2014, so he could inherit the title the 8th Earl of Lucan.
Speaking after the judgement Bingham said he was "very happy", adding that the decision had been "a very long time coming" and that he would be accepting the title immediately, Sky News reported.
Lord Lucan vanished after Sandra Rivett, nanny to his three children, was found murdered at the family home at 46 Lower Belgrave Street, central London, on November 7, 1974. Bingham was seven at the time.
Lord Lucan disappeared on November 7, 1974 after his nanny was found murdered
Even though Lord Lucan was officially declared dead by the High Court in 1999, there have been reported sightings in Australia, Ireland, South Africa and New Zealand, and even claims that he fled to India and lived life as a hippy called "Jungly Barry".
Lord Bingham had said the 1999 declaration had not proved death "for all purposes" and that the new law allowed for a "more complete process".
High Court judge Mrs Justice Asplin granted Lord Bingham the death notice, on the basis she was satisfied Lord Lucan had not been known to be alive for at least seven years. She said none of his family or closest friends had seen or heard from him, or had reason to believe he was alive.
Even though Lord Lucan was officially declared dead more than 15 years ago, there have been reported sightings in Australia, Ireland, Africa and New Zealand.
Lord Bingham lawyer Michael Bloch QC said the Metropolitan Police had not given credence to any of them.
Rivett's son Neil Berriman said he was sure Lord Lucan was "guilty of something" and that he believed the peer escaped in the aftermath of his mother's killing.
Speaking outside court, Sky News reported him as saying: "I personally, along with other people I worked with on this case, know that the convenient drowning, shooting that night, of Lord Lucan is not true."
He said he believed police had an internal document stating Lord Lucan was possibly alive in 2002 onwards.
Berriman: "This document also confirms the blood contamination - which I am grateful for George Bingham's statement, he has made.
"What we have here - and I would like to take the time to express my feelings - is various people withholding evidence and not telling the truth."
Lord Bingham expressed his sympathy for Berriman and his family, describing Rivett as a "lovely lady".
The boradcaster quoted him as saying: "Our family has no idea how our own father, my father, met his own end and whether he did so at his own hand or the hand of others on that fateful evening. It is a mystery, and it may well remain that way forever."
"I would ask, with a very quiet voice, for everyone to consider a person did die here in terrible circumstances, and a family lost a father.
"We, none of us, know actually what happened, nor will we ever. And as a British person, I still prefer to consider a person innocent until proven guilty in a court of law."
On the night of Lord Lucan's disappearance, the nanny's attacker also turned on Lord Bingham's mother, Lady Lucan, beating her severely before she managed to escape and raise the alarm at a nearby pub.
Lord Lucan's car was found abandoned and soaked in blood in Newhaven, East Sussex, and an inquest jury declared him the killer a year later.
George Bingham said he will take his late father's title immediately
Berriman, 47, has previously said: "I don't know if Lord Lucan is alive or not - but I want justice. There have been too many cover-ups already and he should not be declared dead. If Lord Lucan is still alive, he should be prosecuted."
Author Ian Crosby, who has undertaken extensive research into the peer's disappearance, was refused permission to intervene in the case, as his status was that of an expert rather than a family member.
Crosby said the majority of the hundreds of emails he had received could be discounted, but one - from a man who frequented Lord Lucan's old haunt the Clermont Club - merited investigation.
"It appears he might have some information and it may transpire that Lord Lucan actually killed himself on the morning of Friday November 8 1974."
Master Teverson said if Crosby thought the new material was of sufficient importance, he could draw it to Lord Bingham's attention or make a further application to the court.
Lord Lucan was known as "Lucky”’ among the gambling set of friends who bet huge amounts at the Clermont club, John Aspinall’s former casino in London. These friends, it has previously been claimed, spirited him away to West Africa following the murder, and also arranged for his children to be flown out to visit him.
Lord Lucan has also been sighted in India, Australia, Holland and living in a Land Rover in New Zealand with a goat called Camilla, while another tale goes that his body is at the bottom of the English Channel.
Another theory was that he was smuggled out of Britain by an ex-MI5 agent and hidden in a Greek monastery.