It has been officially confirmed that Casey Stoner will miss the next three rounds of the 2009 MotoGP World Championship, ending his title chances.

Ducati's 2007 world champion, who took four podiums (including two wins) from the first five rounds of this season, has been suffering from a mysterious energy-sapping illness in the five rounds since.

Stoner was sick and visibly exhausted after finishing third at Barcelona on June 14 - then struggled with 'severe fatigue' at Assen (third) and Laguna Seca (fourth), prompting extensive medical checks after the US GP.

But those found only 'slight gastritis and mild anaemia', a diagnosis the young Australian was openly doubtful about and he noticed no significant improvement in his physical condition at the following German (fourth) and British events (14th).

Epstein Barr Virus, which can lead to chronic fatigue syndrome, was suggested as one possibility but never proven.

After Donington, Stoner returned to Australia during the summer break to seek consultation from trusted sports doctors.

They are yet to pinpoint the problem, but think a virus during the Catalunya round triggered the fatigue problems. With medical tests continuing, Stoner will withdraw from action until Estoril on October 4 to try and regain full fitness.

"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 was leading the world championship at the start of the Catalunya round, but has now dropped to third in the MotoGP World Championship, 37 points behind leader Valentino Rossi.

In the five races since his illness, Stoner has frequently lost pace during the second half of the (dry) races.

Stoner claimed 90 points in the five races before Catalunya, but only 60 points in the five races since (although his 14th place at Donington Park was caused by a gamble on wet tyres).

Stoner's place at the Ducati Marlboro team will be taken by rookie Mika Kallio, who will move across from the satellite Pramac squad to ride alongside Nicky Hayden.

Although yet to finish higher than eighth this season, Kallio has been a top six contender at several rounds and already looks set to be retained by Ducati for 2010.

"When I took the call I could hardly believe it, but then I talked to my manager and I realised it was true," revealed Kallio, who badly injured a finger at Assen. "I am proud to be racing the next three events with the Ducati Marlboro Team.

"Obviously I'm not expecting to suddenly be a second quicker, because our bikes are very similar to the official machines, but I hope to be able to pick up some good results for the team.

"I would particularly like to thank [Pramac boss] Paolo Campinoti, who has allowed me to enjoy this incredible opportunity. It will be a real honour for me. Nonetheless I would like to send my best wishes to Casey, and I hope to see him back out on track as soon as possible."

Kallio's Pramac seat at Brno, alongside Niccolo Canepa, will be taken by Ducati's factory World Superbike rider Michel Fabrizio. Fabrizio has previously made MotoGP appearances for WCM (2004) and as a stand-in at Gresini Honda (2007).

Fabrizio, who took his first WSBK victory earlier this season at Monza, is currently third in the WSBK championship behind Ben Spies and his team-mate Noriyuki Haga.

The Italian has a previous best MotoGP finish of tenth, which he achieved with both WCM and Gresini.

Stoner's withdraw not only all-but-ends his 2009 title hopes but will also have a massive impact on Ducati, which is heavily reliant on the #27.

The next best Desmosedici rider in the riders' championship is factory team-mate Hayden, in 13th. The American has scored just 47 points compared with 150 for Stoner.

"It's obviously difficult for us to contemplate the next three races without Casey, but it's clear that the most important thing at this time is his health," said Ducati MotoGP project director Livio Suppo. "Since his debut with us in Qatar in 2007, Casey has won 18 times as well as taking countless podium finishes, becoming one of the top riders in the world championship in the process.

"We have already overcome tough times together, like when he had to undergo surgery on his scaphoid towards the end of last season. We are united whether in times of success or in times of difficulty. We are confident that the doctors will help solve the issues that have affected him since early June and that Casey will be able to take part in the last races of the year in peak fitness. Then we can prepare for 2010 in the best way possible.

"A big thank you to Paolo Campinoti who as always has proved to be a great supporter and an ideal partner for Ducati, allowing Mika to ride with the Ducati Marlboro Team for the next three races. Mika is having a really positive debut year in MotoGP and we are delighted that he will be alongside Nicky in the official works team.

"In his place at Brno in the Pramac Team will be Michel Fabrizio, who I am certain will know how to give the right kind of performance on one of his preferred tracks," Suppo concluded.

Stoner's Brno exit also means that he will not take part in the Brno post-race test, the last opportunity for track time outside of a grand prix weekend before the end of the racing season. The outing is usually used to test ideas for next year's motorcycles.

The 2009 MotoGP World Championship now looks like a direct fight between Fiat Yamaha team-mate Rossi and Jorge Lorenzo, with fourth placed Dani Pedrosa (Repsol Honda) a massive 72 points from the top.

Rossi starts this weekend's Brno round 25 points clear of Lorenzo.

Stoner crashed out of the lead of last year's Czech Republic Grand Prix, while being pursued by Rossi.