The 28-year-old, nicknamed “Thorpedo,” quit competition in 2006 after a glittering career in which he captured nine Olympic medals — five gold, three silver and a bronze — at the 2000 and 2004 Games along with 11 world championship titles.
“I never thought I would be swimming in a competitive way again but I am glad that I am,” said Thorpe. “I have spent four years away from the pool and I needed those four years.
Thorpe said he had registered for the drug-testing regime on Wednesday as he was required to do nine months before beginning competition.
At the height of his career, Thorpe was the greatest middle distance freestyler of all time, he was never beaten in the 400 meters and only once over 200, but he said he would now target the relays and the 100 and 200 freestyle.
“I’m in the process of turning myself into a sprinter who can also maybe do a good 200 as well,” he said.
“I don’t think in the short time that I have that I’ll be able to go through enough training cycles to be able to prepare myself for the (400).”
Thorpe said he would, however, swim in an individual event in London if he qualified at Australia’s Olympic trials in early 2012, but cautioned his compatriots to be realistic about what he could achieve.
“If I had an individual swim as well, I would take it,” he said. “But it still is baby steps ... just because I’ve had extraordinary results before, doesn’t mean I will instantly be able to have them again.
“The level of expectation that’s on me is enormous, it’s probably only outweighed by my own expectations. I don’t want to let people down and I don’t want to let myself down.”
Intrusions into his private life had robbed him of his enjoyment of the sport four years ago, he said, and he asked that the media respect his privacy even if he would be training mostly in Abu Dhabi and Europe.
Thorpe said he made the decision to return in September as the London Games would be the last he would physically be able to contest, but did not rule out continuing after 2012.
“I went to see the swimming venue for the London Olympics and it’s an extraordinary venue and I could taste it, which is something I hadn’t felt about swimming for a very, very long time,” he said.
“It hasn’t been something I’ve taken lightly.”
Admitting that he had asked everyone who knew to “lie through their teeth” to protect the secret, Thorpe said Australia head coach Leigh Nugent had been overseeing his training regime mostly by text message.
“To hear Ian publicly declare his intention to return to the pool today, with the aim of further Olympic success in London, is a massive boost for swimming in Australia and our aim of returning to the top of world swimming,” Nugent said.
“Now that he’s confirmed his intention to compete, we can really increase his training and make good steps toward further success.
“We know he is an outstanding athlete, and with application and commitment he will make a success of this comeback.”
The high tech bodysuits Thorpe wore in his heyday have now been banned but he said his feeling had always been that they were not that significant a factor.
“I think a lot of it comes down to the performance of the athlete,” said Thorpe. “I used to swim in the smallest costumes you’ve ever seen and I’ve swum in the longest. I was able to break world records in both.”
Although a representative of a major corporate sponsor sat beside Thorpe during the news conference, the swimmer denied that financial matters had inspired his return.
“I am not motivated by money, if I was, I would never have stopped,” he said, adding: “I have had an almost flawless career and I’ve put this at risk.”
