Champion jockey Richard Hughes is to retire earlier than planned at the Goodwood Festival in two weeks.
The 42-year-old had already announced this would be his final season but said his move into training had hastened his decision to quit.
"To be a champion jockey and to be successful you have to be totally committed," he said.
"It's become clear in the last two weeks that I've realised what that [training] will entail."
Three-time champion Hughes is poised to embark upon a training career as early as the autumn.
|BBC's horse racing correspondent Cornelius Lysaght|
|"Hughes had said he'd be winding down riding commitments when the sales season - all-important for trainers seeking clients - got under way.|
|"But that's from late August/September so this news is in one respect a surprise; equally though Hughes is a smart operator who wouldn't want to jeopardise any plans, so from that point of view it's no shock.|
|"Meanwhile, with no Hughes for the final weeks of the title race, and probably no Ryan Moore because of injury, Silvestre de Sousa's odds on being champion jockey have shortened again.|
He said: "I have always given 100 per cent in everything I've done and I don't want to stop doing that now.
"I always knew this was going to be my last year, and I'm comfortable with the decision [to retire after Goodwood].
"Whenever I was going to retire I was always going to miss out on riding good horses. Now is the time."
Writing in his Racing Post Saturday column, he added: "If you have backed me to win the title, I can only apologise sincerely.
"I really am sorry things have not panned out as I expected. The scale of what it is to come is bigger than even I had imagined."