Marco Simoncelli (left) finished second in last week's Australian MotoGP behind Casey Stoner
"It is official. He is dead. He succumbed to his injuries,'' said an official with Dorna, the company that runs MotoGP, who spoke on condition of anonymity pending a formal announcment.
Simoncelli, 24, had his helmet knocked off in the accident, which occurred when the Italian lowsided from his bike at turn 11, before falling over and being hit by Colin Edwards and Valentino Rossi.
His bike veered across the track and straight into the path of Edwards and Rossi. The race was immediately red-flagged.
Simoncelli, who was motionless on the track after the impact, had been fourth at the end of lap one.
Edwards also fell but escaped serious injury, while Rossi, on a Ducati, was able to return to the pits.
Newly-crowned champion Casey Stoner admitted watching the accident made him worried for Simoncelli.
"As soon as I saw the footage it just makes you sick inside," the Australian told BBC2.
''Whenever the helmet comes off that's not a good sign."
