Kieren Fallon has retired from the saddle after suffering with depression for "the best part of three years".
The Irish Turf Club's chief medical officer Dr Adrian McGoldrick, speaking on the 51-year-old's behalf, said the illness had gone unnoticed when he was riding in England and America.
County Clare-born Fallon has returned to his homeland this season, and has now sought medical advice to help combat "quite profound depression".
Dr McGoldrick said: "I first became aware of it when he came to see me for his licence earlier this year and he was obviously very significantly depressed.
"Kieren's had quite significant depression ongoing for the best part of three years which has gone undiagnosed in England and America.
"It got worse and I met with him on Sunday and have arranged to have it managed.
"He went to see a specialist in America and nobody picked up on it.
"It's quite profound depression. As soon as I can get a bed organised for him, he'll be going to hospital here in Ireland.
"Hopefully we can get him managed and get him ready for the next stage of his life.
"He said he won't be returning to race riding afterwards and will move on to another phase of his career, whatever that might be.
"He felt himself he had no motivation for the last two or three years and that had affected his depression. At this stage of his life he feels he has to move on.
"We know that a lot of elite athletes have depression. I commissioned a survey in racing last year and 49 per cent of jockeys in Ireland actually had symptoms of depression."
Fallon, a six-times champion jockey in Britain, will continue to play a part at the yard of young Curragh trainer Michael O'Callaghan, with whom he has been attached since his return from America in the spring.
O'Callaghan said: "He had a fall on the gallops last week and he just said he's 51 now and doesn't bounce like he used to.
"Kieren has been a great asset to have around the yard.
"He rode his first Group winner for a long time for us not too long ago.
"It's been great to have him here and he is going to remain here as a work rider and advisor, hopefully for a while to come - he's just giving up the race riding.
"He's had an amazing career on the track - he must be one of the best jockeys of all time.
"He is worth his weight in gold to us here, but the main thing is that we just want what is best for Kieren."
The three-times Epsom Derby winner's last ride in public came on June 26, when he rode O'Callaghan's Magical Fire to finish fifth at the Curragh.
Fallon was widely regarded as a master of his trade in his pomp and claimed 16 Classic victories in Britain and six in Ireland.
During his halcyon years, he was also attached to powerhouse yards of the late Sir Henry Cecil, Sir Michael Stoute and Aidan O'Brien.
His career was often blighted by controversy, though, and in October 2007 he was embroiled in a two-month-long corruption trial at the Old Bailey.
Fallon was unable to ride in Britain until the end of the trial, but he was cleared of all charges in December of that year.
But one month later he was given an 18-month worldwide ban from racing after he tested positive for a banned substance - the second time he had been suspended for failing a drugs test - following a race at Deauville in August 2007.
Andre Fabre was taken aback by the news and will never forget the day Fallon won the Prix de l'Arc de Triomphe for him on Hurricane Run in 2005, as well as the Irish Derby that year.
"He won for me an Arc that was not impossible to win, but it was thanks to him," said the French training great.
"I liked the way he used the horses and I liked the person as well. He's a character and a nice man. I hope he's going to be okay.
"Riding has been his life. He was top class."
Derby-winning trainer Peter Chapple-Hyam said he could sympathise with Fallon as he has been hit by bouts of depression.
"It came as a bit of suprise. Obviously he was a world-class jockey. He's done everything he can achieve and good luck to him whatever he does," he told At The Races.
"I've read about his depression. I go through all that, so I know what it's like. He deserves a good retirement and I'm sure whatever he does he'll do it well."
Former top rider Johnny Murtagh said Fallon was a "brilliant, brilliant" jockey, adding: "I'm just thinking back, one of the best rides I've ever seen in the biggest race in the world - the Derby with Kris Kin.
"The best horse didn't win the race, the jockey on the day carried him home.
"It was a funny race, we were going very slow. Pat Eddery (on eventual second The Great Gatsby) made the running and we were so tight coming down the hill at Epsom, it was unbelievable. We were like sardines squashed together.
"I pulled out on Alamshar and moved to the outside and Fallon went down the inside, so I thought 'that's the last we'll see of him'.
"Lo and behold, the last 150 yards, he's got him home to win. A brilliant, brilliant race and a brilliant Derby - not a brilliant horse, but a brilliant ride."
Frankie Dettori described Fallon as "a ferocious competitor".
Fallon and Dettori developed an enduring rivalry which for many years saw the two star riders battle it out in the top races in Europe.
Dettori said: "I have known and ridden alongside Kieren for the past 30 years.
"He has always been at the top of my list as the most ferocious competitor and, over the years, we have gained great respect for each other as riders.
"It's sad to see Kieren finish his career as a jockey and I wish him all the best in the future."
Fallon replaced Jamie Spencer as Aidan O'Brien's stable jockey in 2005 and many great races came their way, including the 2007 Prix de l'Arc de Triomphe with Dylan Thomas and a host of Classics.
O'Brien said: "Kieren was an exceptional jockey and we had a lot of great days together.
"We would all like to wish him the very best for the future."
Fallon - the man for the big occasion
Kieren Fallon's genius in the saddle was without question. Here, we look at six of his very best rides.
TOP CEES, Chester Cup, Chester (1995)
A big-race winner on the Flat and over jumps, Top Cees was a tough stayer who was trained to the minute to land several big handicaps - and they don't come much bigger than the Chester Cup. But that's only half the story. Fallon had been in the plate on his previous start when an eyecatching failure at Newmarket and comments in the Sporting Life prompted a libel action that was fought out in a high-profile court case three years later. Fallon won that, just as he did the Chester Cup, where he picked his way through the tightly-packed field from the rear for a five-length verdict. Ice-cool.
MEDICEAN, Coral-Eclipse, Sandown (2001)
A Fallon masterclass. Prior to the Eclipse, Sir Michael Stoute's four-year-old, a crack miler, had only tried 10 furlongs once, when supposedly a handicap snip in the Magnet Cup at York. Unfortunately a draw somewhere near the car park put paid to his chances, so stamina was something of an unknown. The Machiavellian colt looked booked for a place at best approaching the final furlong, but Fallon's never-say-die attitude was never more evident as his mount made rapid late progress, swallowing up Grandera's lead and getting home by what was in the end a cosy half a length.
KRIS KIN, Derby, Epsom (2003)
The money was down and punters' pal Fallon delivered just when it mattered most. Sir Michael Stoute's colt was the subject of a massive late plunge for the blue riband and the jockey proved he was a worthy successor to the legendary Lester Piggott as 'King of Epsom' with a vintage ride. Kris Kin was in a reasonable position coming down the straight, but there was still plenty to do as he was pulled off the rail for his run - and an appreciative crowd nearly lifted the roof off as Fallon fired him up for a ground-devouring finish to capture the ultimate prize by a length. Absolutely brilliant.
HURRICANE RUN, Prix de l'Arc de Triomphe, Longchamp (2005)
Andre Fabre's colt made it look easy in the end but it was far from a cakewalk in Europe's most prestigious race as Fallon engaged his razor-sharp brain and fearless spirit to make it happen. Hurricane Run looked in trouble towards the rear as they headed into the straight, but Fallon was unconcerned and dived for the inside berth in a risky manoeuvre that proved a race-winner. His top-class partner did the rest, seizing the openings as they came to launch a blistering finish that carried him home by two lengths. This was a jockey at the top of his game. The master at his most masterful.
FOOTSTEPSINTHESAND, 2000 Guineas, Newmarket (2005)
Not necessarily Fallon's greatest ride - though executed with the big-race precision by now expected - but definitely one of his most important. Fallon's surprise switch from Sir Michael Stoute's yard to take over the retainer at Ballydoyle required a Classic victory for vindication, and Fallon supplied two, winning the 2000 on Footstepsinthesand and the 1000 on Virginia Waters for Aidan O'Brien. It was the first time a jockey-trainer combination had landed both races since 1967. Footstepsinthesand grabbed the lead over a furlong out in the centre of the track and won by a length and a quarter, to Fallon's obvious delight.
DYLAN THOMAS, Prix de l'Arc de Triomphe, Longchamp (2007)
Pressure, what pressure? Less than 24 hours before his trial at the Old Bailey was due to begin, Fallon could have been forgiven if he had wanted to hide away in the shadows, far away from the spotlight, and certainly a long way from the glare of the racing world gathered in the suburbs of Paris. But he was a man with a job to do, and how he did it. The ground had been thought to have gone against the O'Brien-trained Dylan Thomas, with many favouring the chance of his stablemate Soldier Of Fortune. But Fallon conjured his old magic to drive his partner home in a pulsating finish from Youmzain. Interference was caused in the process, however, and the local officials took well over 30 minutes before upholding the result. The crowd went wild afterwards, confirming once and for all Fallon's place in the racing public's affections.