Jockey Pat Smullen is "fully focused" on making a complete recovery after he was diagnosed with a tumour.
The nine-time champion Irish Flat jockey said in a statement that he will not be riding "for the foreseeable future" as he receives medial treatment.
Smullen, 40, 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."
Smullen is stable jockey to the powerful Dermot Weld yard and the partnership teamed up to win the Investec Derby and Irish Derby with Harzand in 2016.
He had been champion jockey for the previous three seasons before being pipped at the post by Colin Keane last year.
Racing was quick to pledge its support to Smullen, who has not ridden since March 16.
Fiona Craig, of owners-breeders Moyglare Stud, said on Twitter: "Devastated - no other words, but also more than aware that there is no more determined fighter in a tight finish than PJS, and all that plus will go into fighting to return to what he loves so much."
Paul Smith, an owner and son of Coolmore parnter, Derrick, tweeted: "My heart and prayers go out to @patjsmullen & @francescrowley (Smullen's wife) & the whole family for a full & speedy recovery. Stay strong Pat."