The Duchess of Cornwall was left "utterly devastated" after her brother died from a s erious head injury sustained in a fall in New York.
Mark Shand, 62, was taken to hospital on Tuesday night after reportedly slipping while lighting a cigarette and hitting his head on the pavement outside an after-party for a charity event .
Camilla and the Prince of Wales, who are preparing for a high-profile tour of Canada next month, are believed to have received the bad news at their Birkhill estate in Scotland.
A Clarence House spokesman said: "It is with deep sadness that we have to confirm that the Duchess of Cornwall's brother, Mark Shand, has today passed away in New York.
"Mr Shand died in hospital as a result of a serious head injury which he sustained during a fall last night.
"The Duchess, the Prince of Wales and all her family members are utterly devastated by this sudden and tragic loss.
"Mark Shand was a man of extraordinary vitality, a tireless campaigner and conservationist whose incredible work through The Elephant Family and beyond remained his focus right up until his death."
Mr Shand was a conservationist and travel writer who was known for his love of adventure and fondness for glamorous women.
He was in New York for an auction at Sotheby's in aid of underprivileged children and his own Elephant Family charity, which aims to save the endangered Asian elephant from extinction in the wild.
It was the finale for the month-long Big Egg Hunt NYC by Faberge, which saw egg sculptures sold to the highest bidder.
The charity event raised 1.6 million US dollars (£950,000) for charity and was also attended by Princess Eugenie, according to reports.
A spokesman for the Elephant Family said: " Today we have lost the head of our family. Mark Shand was a true force for conservation.
"He was both a legend and inspiration and above all our great friend.
"We ask at this time that people's thoughts are with his loved ones. We will miss him always."
Mr Shand, Camilla's only brother, wrote numerous books and featured in documentaries, with elephants and India as recurring themes.
The adventurer's book Queen Of The Elephants won the 1996 Thomas Cook Travel Book Award and the Prix Litteraire d'Amis, and was made into a BBC documentary.
During his youth he was a renowned nightclubber who was expelled from Milton Abbey School in Dorset for smoking cannabis and had stints as a rally driver and bobsleigh competitor.
His father, Major Bruce Shand, decided to send him to Australia after he was expelled from school and he stopped off in India, where his love of elephants first took hold.
Later in life he was romantically linked to the likes of Bianca Jagger, Jackie Onassis's sister Lee Radziwill and Jackie's daughter, Caroline Kennedy.
In 1990 he married former French actress Clio Goldsmith, the niece of billionaire entrepreneur Sir James Goldsmith and the cousin of Jemima Khan and Tory MP Zac Goldsmith.
The couple, who split in 2009, have a daughter together called Ayesha.
He recently denied being in a relationship with Nancy Dell'Olio, the former girlfriend of ex-England football manager Sven Goran Eriksson.
He said: "Let me be clear, I have never had an affair with Nancy Dell'Olio. We have never dated. There is no romance; there has not been any romance, there never will be."
Describing her as a "remarkable character", he explained that he knew Dell'Olio through his charity work.
Tributes poured in on Twitter following the sad news, with heiress and campaigner Jemima Khan posting: ? "RIP Mark Shand: Best man, godfather to my son & my cousin's ex".
Ms Khan's ex-husband, former Pakistan international cricketer and politician Imran Khan, 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."
Television personality Piers Morgan said: " What an awful, tragic thing to happen. Poor Camilla & her family incl @tomparkerbowles."