Former jockey Richard Fox is on a life-support machine after collapsing in Newmarket on Saturday.
The 57-year-old struck his head in the fall and was revived by doctors after his heart stopped.
He is currently in West Suffolk Hospital in Bury St Edmunds where a spokeswoman told the Racing Post his condition was stable.
Friend and racing commentator Derek Thompson said on Twitter: "Pray for Richard Fox."
Thompson added on the social networking site: "One of racing's great characters is very ill in West Suffolk hospital. He is on a life support.
"Foxy collapsed and head injured sat late morning-rushed to hospital-then heart attack-brought back but now on life support in Bury St Edmunds."
Fox has been a beneficiary of the Injured Jockey's Fund since suffering an aneurysm while on the Newmarket gallops in 1999.
The former apprentice of Frenchie Nicholson, renowned for his sense of humour, had a successful lightweight career.
The Lincoln, Cesarewitch, Northumberland Plate, Ascot Stakes and Bunbury Cup are among his handicap victories.