Nine-time Irish champion jockey Pat Smullen is being treated for a tumour and faces a lengthy spell out of racing.
Smullen, who won the 2016 Epsom Derby on Harzand, missed the start of the Flat turf season at the weekend.
Medical tests have shown that his illness is more serious than first thought.
“A tumour was discovered and I won’t be riding for the foreseeable future,” said the 40-year-old.
Smullen, stable jockey for trainer Dermot Weld, has recorded a string of big victories during his career including the 2000 Guineas with Refuse To Bend in 2003 and the Gold Cup at Ascot on Rite Of Passage seven years later.
In a statement to the Racing Post, he said: “Most people will have been aware that I have been in hospital in recent days undergoing tests.
“I was showing some of the symptoms associated with a gallstone problem but it was discovered that the problem was more serious.
“A tumour was discovered and consequently I won’t be riding for the foreseeable future. It’s a new challenge for me and it’s one I am fully focused on overcoming.
“I’ve got tremendous support from family and friends, both within racing and outside of racing, and I’m very positive about making a full recovery.”