Italian rider Marco Simoncelli has died after a horrific crash at the Malaysian MotoGP in Sepang.
The race was stopped after four minutes when Simoncelli's bike veered across the track at turn 11 into the path of Colin Edwards and Valentino Rossi.
Marco was flamboyant on and off the track. When someone dies, everyone always says they loved life. But he had a very vibrant personality. He already had a huge fanbase around the world, partly down to aggressive riding - but also because he was just a cool guy. He didn't take himself too seriously and would have been a big star for next year.Matt RobertsBBC MotoGP presenter
The 24-year-old had his helmet knocked off and was hit by both other riders.
American rider Edwards also fell but escaped serious injury - while fellow Italian Rossi was able to return to the pits.
Simoncelli lay motionless on the track after the impact, while the race was immediately red-flagged.
At first officials were looking to restart the race before the extent of Simoncelli's injuries became clear.
The race was cancelled amid confusion in the grandstand, with fans throwing bottles to show their initial displeasure.
The death is the first fatality in MotoGP since Japan's Daijiro Katoh died from injuries sustained at the 2003 Japanese GP.
BBC MotoGP presenter Matt Roberts said: "Marco lost his helmet in the crash. The officials said that, when the track medics got to him, he was in cardiac arrest. They tried to resuscitate him in the ambulance and the medical centre."
Honda rider Simoncelli entered MotoGP for the 2010 season and won his first pole position at the Catalunya race in June this year.
Simoncelli's first podium finish came in the Czech Republic in August when he finished third but he bettered that with a second-place finish at the Australian GP.
World champion Casey Stoner said: "As soon as I saw the footage it just makes you sick inside. Whenever the helmet comes off that's not a good sign."
British MotoGP rider Cal Crutchlow Tweeted: "RIP Marco Simoncelli! A great rider and all round nice guy. My thoughts are with all his family & friends. I will never forget today."
Australian Formula One driver Mark Webber Tweeted: "RIP Marco. A special talent that will be missed. Thinking of your loved ones and all the MotoGP paddock."
SIMONCELLI FACTFILE
1987, January: Born Cattolica, Italy
2002: Wins the European 125cc title
2008, June: Wins his first 250cc race at the Italian GP
2008, October: Wins the 250cc World Championship
2009: Finishes third in the 250cc World Championship
2010: Moves up to MotoGP class riding for Honda
2011, June: Claims first MotoGP pole at the Catalunya GP
2011, August: First podium finish at Czech Republic
2011, October: 2nd place at Australian GP