The organisers cancelled Sunday's race as soon as the extent of Simoncelli's injuries became apparent.
A statement on the MotoGP official website read: 'On Sunday 23rd October, during the MotoGP race at the Shell Advance Malaysian Motorcycle Grand Prix at the Sepang International Circuit, San Carlo Honda Gresini's Italian rider Marco Simoncelli suffered a serious accident in which he received critical injuries.
Simoncelli, who won the 250cc world championship in 2008, clinching the crown in Sepang, stepped up to MotoGP in 2010.
His death was the first fatality in MotoGP since Japan's Daijiro Katoh died from his injuries sustained at the 2003 Japanese Grand Prix.
Newly-crowned champion Casey Stoner admitted immediately after the crash he had feared 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.'
Tributes poured in for Simoncelli from teams and riders.
British MotoGP rider Cal Crutchlow wrote on Twitter: 'RIP Marco Simoncelli! A great rider and all round nice guy. My thoughts are with all his family & friends. I will never forget today.'
Formula One driver Mark Webber tweeted: 'R.I.P Marco A special talent that will be missed... Thinking of your loved ones, and all the motogp paddock..mark.'
A message on Simoncelli's Gresini Honda team website read simply 'CIAO "SUPER
The Rizla Suzuki team wrote: 'All at Rizla Suzuki are deeply affected by today's tragic events and send their deepest condolences to Marco's family and friends during this time of great loss.
'R.I.P Marco Simoncelli, 20 January 1987 - 23 October 2011 - A true competitor.'
