Nicky Hayden, the 2006 MotoGP world champion, suffered severe brain damage as the result of his bicycle accident, a medical statement issued on Thursday confirmed.
The American remains hospitalised in the Intensive Care unit in the Bufalini hospital in Cesena in Italy after being hit by a car while riding with friends on Wednesday.
In an update issued at 5pm local time on Thursday afternoon, the hospital said 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 have 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 and was his team-mate in '03 and '10-11.
"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."
Hayden left MotoGP at the end of 2015 to move to the World Superbike Championship with Honda, but made a pair of starts as an injury replacement last year.
His team-mate in the second of those, last October's Australian Grand Prix standing in for Dani Pedrosa, reigning world champion Marc Marquez also paid tribute.
"Since 2008, when I joined the Repsol KTM team in 125cc, he was a really nice guy with me," Marquez said.
"In Australia, I spent a lot of time with him. It was really nice, we had an incredible party on the Sunday night.
"I remember that he was going to sleep but then he stayed until six in the morning with me, because he enjoyed it a lot.
"I wish the best for him, but especially all his family."