Simoncelli dies after horrific crash Marco Simoncelli has died from the injuries he sustained in a crash on the second lap of the Malaysian Grand Prix.
He was 24.
Simoncelli lay stricken on the track after the crash, his helmet having come off during the incident.
He was taken by ambulance to the circuit's medical centre, but at 4.56pm local time it was announced he had succumbed to his injuries.
Simoncelli had established himself as one of the most exciting new stars in MotoGP this season, his second year at the top level with the Gresini Honda team.
A former European 125cc champion, Simoncelli was a race winner and frontrunner during three years in world championship 125cc racing before coming to the 250cc series in 2006.
He spent four seasons with the Gilera team, with the breakthrough coming in 2008 when he not only became a winner for the first time, but charged to the title, beating Alvaro Bautista.
Simoncelli stayed on in 250cc to defend his title in 2009, although he narrowly lost the crown to future Gresini MotoGP team-mate Hiroshi Aoyama after an early-season injury left him playing catch-up.
He secured a MotoGP seat with Gresini for 2010 and became ever more competitive as his rookie season went on.
That earned him a factory specification Honda this season, and he took full advantage to get among the frontrunners from the outset.
Pole positions at Catalunya and Assen came amid a run of six straight front row starts, although incidents in races meant he could not deliver the results his speed promised - and led to rows with other riders.
But Simoncelli calmed his style as the season progressed, finally taking his maiden MotoGP podium at Brno, and achieving a career-best second in Australia just a week ago. He had also secured works Honda equipment again with Gresini next season, ensuring he would have started 2012 as a potential MotoGP winner.
Simoncelli's death is the first in motorcycling's premier class since another Honda rider, Daijiro Kato, was killed at the 2003 Japanese Grand Prix.
Shoya Tomizawa also died in a similar crash to Simoncelli in Moto2, the class below MotoGP, last year.
MotoGP bosses and riders have been working hard to improve safety following Tomizawa's death at the San Marino Grand Prix in September 2010 but have acknowledged there is little more they can do given the implicit danger in motorcycling, especially when riders are hit by other bikes.
All Italian sports events on Sunday will observe a minute's silence in memory of Simoncelli, the Italian Olympic Committee said in a statement.
Simoncelli was a big fan of soccer club AC Milan, who released a statement offering their condolences.
Last season's MotoGP world champion Jorge Lorenzo recently suffered a serious injury which could have cost him a finger, underlining the danger of the sport. The Spaniard said: "On a day like this I don't know what to say. Marco, rest in peace."
Nicky Hayden, the 2006 champion who rides for Italian team Ducati, was heartbroken. "It is a really horrible day for all of us," the American said.
"I saw Marco fall. Leaving the corner he lost the back end and probably he didn't manage to regain his balance on the bike. I feel really bad. On the track we are all brothers. Marco, we will miss you so much."
Simoncelli's death completes a harrowing week for motorsport after British IndyCar driver Dan Wheldon lost his life last weekend.
Wheldon died at the Las Vegas Motor Speedway last Sunday after being involved in a 15-car crash.
Drivers questioned the wisdom of running the IndyCar finale on a very fast, high-banked oval which had not staged an event in the series for 11 years.
Others criticised the 34-strong field sprinkled with part-time drivers.