Valentino Rossi has suffered a double leg break in an enduro training accident in Italy, his Movistar Yamaha team have announced.
The nine-time world champion will now undergo surgery on the injury which was sustained on Thursday evening in an incident which happened close to his hometown of Tavullia.
He was taken to the Ospedale Civile di Urbino hospital for tests, where the extent of his injuries were confirmed.
A Yahama statement read: "Following an enduro training accident on Thursday, August 31st, Movistar Yamaha MotoGP star Valentino Rossi was diagnosed with displaced fractures of the tibia and fibula of his right leg.
"The 38-year-old Italian rider will undergo surgery as soon as possible."
Yahama said a medical update woud be issued on Friday morning.
Rossi is currently fourth in the MotoGP standings with six rounds left, sitting 26 points behind leader and compatriot Andrea Dovizioso. The next race is in Misano on September 10.