Italian MotoGP great Valentino Rossi has broken his right leg in a training accident and will undergo surgery as soon as possible.
The accident, while riding an off-road enduro bike in eastern Italy, looks set to end the 38-year-old's title chances for another year.
Yamaha said in a statement that Rossi had been taken to hospital in Urbino, where he was diagnosed with displaced fractures of the tibia and fibula of his right leg.
Yamaha added that Rossi, who is currently third in the overall standings and 26 points behind leading compatriot Andrea Dovizioso, had been riding an off-road enduro bike.
It said a further medical bulletin would be issued in due course.
The next race is at Misano on Italy's Adriatic coast on September 10th.
Italian media reports said the rider had damaged his right leg, which he also broke in practice for the Italian Grand Prix in 2010. Ross returned to MotoGP after an absence of six weeks that time.
"I've heard from him, his leg really hurts and Valentino is not optimistic... it's a mess," the ANSA news agency quoted Rossi's father Graziano as saying.
"I know that they still have to give him an X-ray and they fear it's a fracture. To do the radiography they have to cut off the boot and Valentino told me he feels really bad."
Rossi made motorcycling history in June when he won the Dutch MotoGP round at Assen, a victory that came 20 years and 313 days after the nine times world champion's first in what was then the 125cc category.
No other rider has been a winner across all classes for a longer period of time.
The Italian has 115 grand prix wins across all classes, second only to compatriot Giacomo Agostini's 122 victories between 1964 and 1977.
His 2017 season has already been hit by injury, the Italian spending the night in hospital in Rimini in May after he suffered lesions of the kidney and liver in another motocross fall.
That accident came little more than a week before his other home grand prix at Mugello on June 4.