Hayden, 35, was hit by a car while riding a bicycle near the beach town of Riccione on Wednesday, 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.
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.
His brother Tommy and mother Rose have flown in from the US to be by his bedside with Hayden’s fiance Jackie, Team Honda wrote on its website.
Italian great Valentino Rossi, who leads the championship and is racing in France this weekend, hailed his former team mate as “one of the best friends I have had in the paddock”.
“Come on Nicky, we’re all with you,” he said.
Hayden last raced in the MotoGP championship in Spain in September 2016 as a stand-in for injured Australian Jack Miller at the privately-run Marc VDS Honda team.
He started 216 races between 2003 and 2015, winning three.
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