Australia's most successful Olympian Ian Thorpe says he will retire from competitive swimming after the 2008 Beijing Games at the age of 25.

The five-time Olympic gold medallist has told The Australian newspaper a new seven-year, multi-million-dollar deal with sportswear company Adidas is unlikely to mean he will extend his career to the London 2012 Games.

"It's not out of the question, but at the moment I intend to retire in Beijing," Thorpe is quoted as saying in Friday's edition.

"It (the Adidas deal) is there if I want to go on, but if I decide that I don't want to, I can become an ambassador for Adidas."

Thorpe will contest Australia's Commonwealth Games trials in Melbourne next week.

Thorpe, 23, said he couldn't wait for the March Commonwealth Games to start in Melbourne but admitted he was a little nervous after taking a break from major competition following the Athens 2004 Olympics.

"I am looking forward to racing but I would like to be swimming at the Commonwealth Games now," he said.

"I am a little bit hesitant - it's a bit like riding a bike after you haven't done it for a while.

"I might stand up on the blocks and be terrified or I might be excited."

Thorpe will contest the 100m and 200m freestyle next week.

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