The Duchess of Cornwall's brother, Mark Shand, has died as a result of a serious head injury he sustained during a fall.
The 62-year-old was in New York when the incident happened on Tuesday night.
"The duchess, the Prince of Wales and all her family members are utterly devastated by this sudden and tragic loss," Clarence House said.
He was described as a man of "extraordinary vitality, a tireless campaigner and conservationist".
Mr Shand was chairman of the Elephant Family, a movement which exists to save the endangered Asian elephant.
In its statement, Clarence House said it was with "deep sadness" that it had to confirm Mr Shand's death.
It said his "incredible work through the Elephant Family and beyond remained his focus right up until his death".
The death comes as the Duchess of Cornwall and the Prince of Wales, currently understood to be in Scotland, prepare for a high-profile tour of Canada next month.
Mr Shand, the duchess's only brother, reportedly slipped while lighting a cigarette and hit his head on the pavement outside an after-party for a charity event.
'Legend and inspiration'
According to the New York Police Department, the emergency services picked him up where he fell outside the Rose Bar and took him to Bellevue Hospital, where he was pronounced dead on Wednesday.
Mr Shand married French actress Clio Goldsmith - the niece of billionaire entrepreneur Sir James Goldsmith and the cousin of Jemima Khan and Tory MP Zac Goldsmith - in 1990. They have a daughter together called Ayesha, aged 19.
Ms Khan tweeted her condolences: "RIP Mark Shand: Best man, godfather to my son & my cousin's ex."
Her ex-husband, former Pakistan international cricketer and politician Imran Khan also took to the social media site to pay his respects.
He tweeted: "Shocked & saddened to learn of the death of one of my closest English friends, Mark Shand, in New York where he was raising funds 4 charity."
Band Aid co-founder and former Ultravox frontman Midge Ure also tweeted: "Dreadful news about Mark Shand. I worked with him on his Elephant charity in Edinburgh. A true gentleman."
The conservationist and travel writer had been in New York for a charity auction at Sotheby's, which was in aid of unprivileged children and the Elephant Family.
The event was the finale of the month-long Big Egg Hunt NYC by Faberge, where egg sculptures were sold to the highest bidder.
It raised $1.6m (£950,000) for charity and was reportedly also attended by Princess Eugenie.
Dan Bucknell, head of conservation and campaigns at the Elephant Family, said Mr Shand had "infectious enthusiasm" and was the "lifeblood" of the organisation.
"He was a very fun-loving, warm-hearted, generous guy," he said.
He said people at the Elephant Family were "still coming to terms" with Mr Shand's death, and said it was vital that "we do him proud" and "take the charity on to greater heights in his name".
Mr Shand featured in documentaries about elephants and India.
He wrote numerous books - one of which, Queen of the Elephants, won the Prix Litteraire d'Amis and was made into a BBC documentary.
As a teenager, the adventurer was expelled from Milton Abbey School in Dorset for smoking cannabis.
His father, Major Bruce Shand, sent him to Australia following his expulsion. On his way, he stopped off in India, which is where his love of elephants first took hold.