Marlboro Ducati's Casey Stoner has been forced to withdraw from the next three MotoGP races, including this weekend in the Czech Republic, because his illness still hasn't been correctly diagnosed.

This means the Aussie has handed the title to the Fiat Yamaha team and now Valentino Rossi and Jorge Lorenzo will battle it out for the riders' championship this year.

“After five extremely difficult races due to my health, I returned to Australia to visit the sports doctors who have looked after me for many years. We have taken the difficult decision not to contest the next three rounds of the championship, to allow my body time to recover from the recent stress," said Stoner.

"The doctors believe that during the Barcelona race I was suffering from a virus, and, that I subsequently pushed my body too hard, leading to problems that have caused my fatigue since then. The doctors are continuing with many tests to try to understand these problems and make sure it does not happen again.

"I have spoken with Ducati and thank them for their understanding at this time. I feel very sorry for the factory, my team, my sponsors and the fans and I am also disappointed because the bike in the last races has been very competitive. I will be doing everything possible to come back at full strength for Portugal."

Stoner will be replaced by Mika Kallio.