Hayden suffered severe head and chest trauma along with several fractures in the accident.

The 35 year-old's condition was not stable enough for surgery to be performed to relieve the pressure on his skull caused by cerebral edema.

He remained in a critical condition in intensive care in the Bufalini Hospital through the weekend.

The hospital issued a statement on Monday from the Hayden’s family confirming that had died.

"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," his brother Tommy Hayden said.

"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.

"Apart from these 'public' memories, we will also have many great and happy memories of Nicky at home in Kentucky, in the heart of the family. We will all miss him terribly.

"It is also important for us to thank all the hospital staff for their incredible support – they have been very kind.

"With the further support of the authorities in the coming days we hope to have Nicky home soon."

Hayden was 2006 MotoGP champion with Honda. The American made a total of 217 MotoGP starts between 2003 and '16 before switching to the World Superbike Championship. He had been 13th in the 2017 World SBK after five rounds.

Nicky Hayden in action in Phillip Island's SBK round earlier this year. Pic: Mpix