Valentino Rossi will miss his home grand prix at Mugello after sustaining a fractured right shin bone in a violent crash in final practice this morning.
The reigning champion, who had already been struggling with shoulder injuries following a motocross crash in April, had a high side at the Biondetti esses 15 minutes from the end of the session.
Rossi landed awkwardly on his right leg and was clearly in agony in the gravel trap. He had to be taken away from the scene on a stretcher.
"Valentino Rossi has suffered a displaced and exposed fracture of his right tibia after crashing in practice for the Italian Grand Prix in Mugello this morning," said a statement from the Yamaha team.
"The reigning world champion crashed his Fiat Yamaha M1 at Turn 13, the fast chicane at the top of the hill, and was taken immediately to the circuit medical centre for x-rays.
"Further information will be made public as soon as a more detailed examination has been completed.
"Rossi will not take any further part in this weekend's Italian Grand Prix."
This will be the first time in Rossi's MotoGP career that he has had to miss a race due to injury, and is a severe blow to his 2010 title hopes, with his team-mate Jorge Lorenzo already holding the points lead thanks to two wins from the three races held so far this year.
AUTOSPORT understands that Rossi will be taken to the Central Traumatological Hospital in Florence for surgery this afternoon, where his open wound will be operated on by Dr Roberto Uzzi. He has been flown to Florence's nearby Careggi Hospital for a thorough assessment first.
Medical sources at Mugello have suggested that the normal recovery period for such an injury would be two months - but have pointed out that fellow riders Toni Elias and Max Sabbatani were able to ride again within 40 days following similar accidents. However even such a rapid recovery would potential rule Rossi out of at least four races.