The American remains hospitalised in the Intensive Care unit in the Bufalini hospital in Cesena, having been hit by a car while riding with friends on Wednesday.

In a 17:00 CET update, the hospital confirmed that Hayden's condition remains extremely critical.

A statement said the 35-year-old suffered serious polytrauma with subsequent serious cerebral damage in the accident and that his prognosis remains unknown.

Hayden's fiancee Jackie has been by his side so far, and she has been joined by his mother Rose and his brother Tommy, who had flown from the United States.

MotoGP legend and Hayden's friend Valentino Rossi paid homage to the American rider, who beat him to the MotoGP title in 2006.

"Very, very bad news from yesterday," said Rossi ahead of the French Grand Prix on Thursday. "I spoke straight away with a doctor, my friend, and he said that from the very first moment the situation was very difficult for Nicky.

"And now with a bit more time you start to understand. It's a great shame because Nicky is a great rider for sure, a great talent, a world champion and always at the top, but especially a very good guy.

"Coming from a good family of racers, from his father to his two brothers, always a good guy, always smiling, so for sure it's very bad. I think the situation is very bad but everybody, we hope for the best."