American MotoGP rider Ben Spies announced on Tuesday his departure from the Yamaha works team at the end of the season, leaving a vacancy alongside championship-leading Spaniard Jorge Lorenzo.
Spies sent a link from his Twitter account (@benspies11) to a story on the Superbike Planet website which quoted him saying via email that he had chosen to leave "for a litany of reasons".
"I will explain more when I feel it is appropriate," he added. "I'm discussing my situation with my sponsors and they are behind me."
Spies, preparing for this weekend's U.S. Grand Prix at Laguna Seca in California, added in a separate message that next year would start a new chapter.
The Tennessee rider, a former world superbike champion, is in his second season at the works Yamaha team but has been outshone by Italian Andrea Dovizioso and Britain's Cal Crutchlow at the non-factory Tech3 Yamaha team.
While Lorenzo leads the standings with 185 points, Spies is 10th on 66.
(Reporting by Alan Baldwin in London, editing by Clare Fallon)