Arrogate, twice crowned the world's best racehorse, has been euthanised after suffering from an unspecified illness.
The seven-year-old retired at the end of 2017 having won over £13.6million in prize money, with notable victories in the 2016 Breeders' Cup Classic and both the Pegasus World Cup and Dubai World Cup in 2017 when trained by Bob Baffert.
He retired to owner Prince Khalid Abdullah's Juddmonte Farm in Kentucky where he was standing his third season before being taken ill last week.
A statement from Juddmonte said: "It is with great sadness that Juddmonte confirms that Arrogate was euthanised this morning, after a battle to save him after an illness that began last week. It is still unclear what the illness was and a post-mortem is currently being carried out.
"Arrogate had a very successful breeding season this year until last week when his breeding was suspended due to an initial suspected sore neck.
"Later in the week, he fell to the ground in his stall and efforts to get him back up proved unsuccessful. He was transported to the Hagyard Clinic under the care of Dr Bob Hunt and Dr Nathan Slovis with Dr Steve Reed representing the insurance company.
"They worked tirelessly with the staff from the Hagyard Clinic eliminating all the obvious causes and performing all available tests, including spinal tap evaluation, X-ray, ultrasound, CT Scan and many blood tests.
"Unfortunately, Arrogate was still unable to stand after four exhausting days for him. His will to fight, so valuable to him on the racetrack, became a challenge in his care. When serious secondary health issues set in, the decision was made to put him to sleep."
Douglas Erskine Crum, CEO of the Juddmonte Group, paid tribute not only to the horse, but also those who had cared for him.
He said: "This is very hard on everyone at Juddmonte, most especially on Steve Dotsey, stallion manager, and Jorge Chavarria who cared for Arrogate daily and who had such a great rapport with him.
"It was a bold decision to ask Bob Baffert to purchase and train top dirt horses for Prince Khalid and, assisted by Donato Lanni, the selection of Arrogate was one of the success stories of the decade on the racetrack.
"Arrogate has bred three crops at stud, has his first yearlings to present to the sales this year and his physical and personality traits are passing very strongly to his offspring."
Garrett O'Rourke, general manager of Juddmonte USA, added: "We will never forget the ride Arrogate took us on.
"Those four amazing races established him as one of the great racehorses of our time, breaking two track records, taking down champion California Chrome in a battle for the ages and then running by Gun Runner in another worldly performance in Dubai.
"We hope that the heartbreak of losing him so young can be soothed by proudly watching the achievement of his runners."