MotoGP world champion Valentino Rossi said he was moving to Italian team Ducati because his work with Yamaha was finished and they no longer needed him.
Rossi, one of Italy's most famous sports personalities, will race for Ducati next year on a two-year contract. He has been with Yamaha since 2004.
He told reporters on Monday at the Czech circuit in Brno, scene of Sunday's Grand Prix:
"I'm happy, I'm very curious and very motivated. This new challenge, I think, will be very interesting - especially because I think that at Yamaha my work is finished."
Rossi added: "We changed the situation from 2004 a lot. Yamaha have great riders so it looks like they don't need me any more."
Clear championship leader Jorge Lorenzo of Spain also rides for Yamaha.
A vacancy at Ducati arose after Australia's Casey Stoner said last month he would be leaving for Honda next year.
Looking forward to 2011, Rossi said: "I think an Italian rider on an Italian bike is great for all the Italian fans and I think we will have a lot of cheering from all the Italian fans."
Yamaha managing director Lin Jarvis added: "Unfortunately he's made finally the choice to go and ride for Ducati next year so it was a sad day yesterday because although we've known about his decision for some weeks nevertheless when you make the final public announcement it becomes a fact."