According to reports, the Red Bull Honda World Superbike rider suffered severe head and chest injuries after being hit by a Peugeot while cycling near the town of Rimini this past Wednesday, following the championship's fifth round held the previous week at the nearby Imola Grand Prix circuit.
He was taken to hospital where doctors judged his condition to be unsuitable for an operation in order to relieve pressure on his skull caused by a cerebral edema.
"On behalf of the whole Hayden family and Nicky's fiance Jackie I would like to thank everyone for their messages of support - it has been a great comfort to us all knowing that Nicky has touched so many people's lives in such a positive way," Autosport quoted Hayden's brother Tommy as saying.
"Although this is obviously a sad time, we would like everyone to remember Nicky at his happiest - riding a motorcycle. He dreamed as a kid of being a pro rider and not only achieved that but also managed to reach the pinnacle of his chosen sport in becoming world champion. We are all so proud of that".
Nicknamed the Kentucky Kid, Hayden made 217 MotoGP starts between 2003 and 2016, with the highlight being his 2006 title for the factory Repsol Honda team five points ahead of then seven-times world champion, Valentino Rossi.
Two further seasons with Honda resulted in a third and second place finish, before a switch to the works Marlboro Ducati outfit for 2009 netted three consecutive third place finishes followed by a fourth in 2012 and fifth in 2013. A move to the private Aspar Honda team for the 2014 and 2015 seasons however saw Hayden record a disappointing 8th and 11th place championship finish, resulting in him departing for the World SBK the next year.
He briefly returned to the MotoGP circus filling in for injured Marc VDS Honda rider Jack Miller at Aragon in round 14, and for Repsol Honda's Daniel Pedrosa at the Australian Grand Prix two rounds later.
Prior to his death, Hayden had laying 13th in the championship with 40 points following three retirements and a best finish of seventh during the second race of the second round at the Chang International Circuit in Thailand.