Former MotoGP champion Nicky Hayden spent a second day in intensive care today after colliding with a car while cycling in eastern Italy, the American's Honda World Superbike team said.

Doctors placed the 2006 MotoGP champion into a medically induced coma, the BBC reported.

"Following the incident, Nicky was treated on site by medical staff and then taken by ambulance to a hospital near Rimini for immediate treatment," Red Bull Honda said in a statement.

"Once Nicky’s condition was stabilised, he was transferred to Maurizio Bufalini Hospital in Cesena and remains in the hospital’s intensive care unit."

The 35-year-old's fiancee and team members were at his bedside, according to the statement.

Italian media quoted the hospital as saying that Hayden had suffered a cerebral edema and lower body fractures and surgery had not been possible immediately.

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.