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MotoGP world champion Casey Stoner will retire at the end of the season after explaining he has lost his passion for the sport.

The 26-year-old Australian had recently denied rumours he was to end his career, but revealed his intention ahead of the Le Mans French Grand Prix.

"This sport has changed a lot and it has changed to the point where I am not enjoying it," he said.

"I don't have the passion for it and so at this time it's better if I retire."

He added: "There are a lot of things that have disappointed me, and also a lot of things I have loved about this sport, but unfortunately the balance has gone in the wrong direction.

"After a long time thinking, a lot of time talking with my family and my wife, this has been coming for a couple of years now, but... I will not be racing in the 2013 Championship."

Stoner recently brushed off rumours that he was contemplating retirement, claiming that "everyone seems quite good at making up stories".

It meant his change of mind came as a big surprise to rival Valentino Rossi.

"It's bad news for the world of MotoGP," said the seven-time world champion.

"At the end of the season, we are going to lose a great rider and a great rival, but it's his decision."

The Italian, 33, dampened speculation over his own future in the sport by claiming he wanted to continue for "the next two years for sure".

Stoner did not reveal his future plans beyond "going forward in different things in my life".

The Repsol Honda rider, who also won the world title in 2007, leads this year's championship by a single point from Yamaha's Jorge Lorenzo after claiming victories in Spain and Portugal.

Despite the strong start to his title defence, Stoner has been critical of technical changes to MotoGP that have increased engine sizes to 1000cc from 800cc and preceded the departure of manufacturers such as Kawasaki and Suzuki from the sport.

He has also suffered with health problems during his time in MotoGP.

Marc Marquez, the Moto2 series leader and Repsol protege, is the likeliest candidate to fill the vacancy left by Stoner, although Jorge Lorenzo is out of contract at the factory Yamaha squad at the end of the season and may also enter the reckoning.