A mystery illness has forced Australia's Olympic and world swimming champion Ian Thorpe to pull out of this month's Commonwealth Games in Melbourne, robbing the event of its biggest drawcard.

His eyes glazed and his throat croaking, Thorpe revealed at a news conference that he had been suffering from bronchitis and a viral infection since February but doctors were unable to diagnose the exact problem.

"It was difficult coming to terms with not being able to compete but in the end it was the only decision I could come to," Thorpe said at the conference, broadcast live on Australian television.

Elite athletes are especially susceptible to infection because their intense training weakens the body's immune system and they are unable to use medication because the most effective drugs are on the banned list.

A five-time Olympic gold medallist, Thorpe said he cut back on his training in an attempt to shake off the bug but had resigned himself to missing the Games.

"I'd struggled for so long and I was trying to push myself to be at a stage where I could compete -- not be up to scratch but be a part of the team.

"But it wouldn't have been a good performance by me if I did compete... I would have been of little, if any, benefit to the team.

"I would have done more damage than good if I had competed."

Thorpe had been planning to make his international comeback at the Commonwealth Games from March 15-26 after taking a break from the sport following the 2004 Athens Olympics [Images].

The 23-year-old missed last year's world championships in Montreal and was eager to return to the pool in Melbourne.

RECORD ATTEMPT

Thorpe had won four gold medals at the 1998 Commonwealth Games in Kuala Lumpur, then a record six at Manchester four years ago and needed just one more to become the most successful athlete in Commonwealth Games history.

The next Commonwealth Games are in New Delhi in 2010 but Thorpe is unlikely to be competing then after recently hinting that he wanted to retire after the 2008 Beijing [Images] Olympics, where he will be bidding to become the first male swimmer to win individual titles at three Olympics.

"I just hope that this isn't something that is recurring," he said. "I hope I have preparations where I am healthy and can produce my best."

The Commonwealth Games were also shaping as a watershed in Thorpe's career and the start of the long road he hoped would lead to China and swimming immortality.

He is already the greatest middle-distance swimmer the world has seen but was planning to use the Games as part of a bold experiment into uncharted waters as a specialist sprinter, entering the 100m and 200m freestyle and three relays in Melbourne.

Australia's head coach Alan Thompson said Craig Stevens, a close friend and training partner of Thorpe, had been added to the team to replace Thorpe but would swim the 400m and 1500m instead.

Stevens famously gave up his spot to Thorpe in the 400m at Athens after Thorpe was disqualified at the Australian trials when he slipped off the blocks. Thorpe went on to win the gold in Athens, and dedicated the victory to Stevens.

MASSIVE BLOW

"I'm just very grateful for this opportunity but the most important thing at the moment is Ian's health and I'm just hoping he can get over it," Stevens said.

Thorpe's withdrawal is a massive blow both to the Games organisers and the Australian men's swim team after Grant Hackett, the world and Olympic champion over 1500m, was forced out with a shoulder injury.

"This is a huge loss to the team," Thompson said. "But whenever one door closes, another one opens.

"Of course you would rather have these two in the team but there's a lot of young fellows waiting to seize their opportunity."