Former MotoGP world champion Nicky Hayden suffered serious trauma and cerebral damage after a cycling accident in Italy, it has been confirmed.

Honda World Superbike rider Hayden, who won the MotoGP title in 2006, was hit by a car while training on a bicycle along the Rimini coast on Wednesday.

He remains in intensive care at the Maurizio Bufalini Hospital in Cesena with his fiancee by his side and has been joined by his brother and mother, who arrived from the United States on Thursday.

"Nicky Hayden's clinical condition remains extremely critical," the hospital said in a statement.

"The young man, who is still in the intensive care unit of Cesena's Bufalini hospital, has suffered serious multiple traumas with subsequent serious cerebral damage.

"The prognosis remains unknown."

Hayden had raced in the Italian round of the Superbike World Championship at Imola last weekend.

A veteran of 218 MotoGP races, he won the world title with Repsol Honda in 2006 - ending Valentino Rossi's streak of five successive championship victories - and also raced for Ducati before switching to the Superbike circuit last year.

He was initially taken to a local hospital in Rimini following the incident before being moved to Cesena.