Doctors in Italy have confirmed former Moto GP world champion Nicky Hayden has suffered serious brain damage following a cycling accident on Wednesday.

Hayden, 35, was hit by a car while riding a bicycle near the beach town of Riccione, three days after racing in Italy's Imola track for the Superbike World Championship.

"Nicky Hayden's clinical conditions remain extremely serious," the Bufalini hospital of Cesena said in a statement.

"He is still in intensive care and suffered a severe polytrauma, resulting in very serious brain damage," the hospital added.

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The American is known in the sport as The Kentucky Kid. He won the MotoGP championship in 2006 and has been competing for Team Honda in the Superbike category since last year.