Current Irish riding champion Jamie Spencer has quit his position as stable jockey for trainer Aidan O'Brien, leaving behind one of the highest profile jobs in European racing.Coolmore spokesman Richard Henry confirmed Spencer's departure in a statement released Sunday after much speculation about the jockey's future."Jamie is a young jockey with a lot of talent and has a long career ahead of him. All the team at Ballydoyle wish him the very best," the statement read.Spencer joined O'Brien as lead stable rider in November 2003, replacing Mick Kinane, who had been part of the Ballydoyle team for five years.Spencer, 24, underwent severe criticism in the press for some of his rides for O'Brien, including a pair of misadventures aboard Powerscourt in the United States. Spencer was blamed when Powerscourt was disqualified from victory in the Arlington Million (gr. IT) and for making an early move with him in the Breeders' Cup Turf (gr. IT). Other rides aboard One Cool Cat and Antonius Pius also drew biting attention.But Spencer also produced group I wins for O'Brien during an off year for the giant racing operation and managed to win the national jockey title, making hime the second-youngest rider ever to do so.