Usain Bolt of Jamaica (left) runs with his teammates during a training session for the IAAF World Championships in Daegu on Tuesday. Max Rossi / Reuters

Usain Bolt, the world's fastest man, leads a powerful group of Jamaican sprinters into the World Athletics Championships but a repeat doping positive by Steve Mullings has cast a steroid shadow over the squad.

Bolt, the reigning world and Olympic double sprint champion, has lost only twice in his professional career and will be favorite to defend his 100m and 200m crowns in South Korea, and seal the same status for the 2012 London Olympics.

"I'm Olympic and world champion. My goal is to defend my titles," Bolt said. "I want to be a legend of the sport and if I do that, it will make me one."

But Mullings, Jamaica's 200 champion with Bolt sitting out the June national meet and third-fastest in the 100, tested positive for a banned substance at the Jamaican championships after serving a two-year steroid ban from 2004-2006.

If his "B" sample is also positive, it could bring a life ban for the 28-year-old training partner of injured US star Tyson Gay, who will not compete in Daegu.

Bolt did not run at the Kingston meet because he was assured of spots in the worlds as defending champion.

Asafa Powell, who won the Jamaican 100 crown in Bolt's absence, and 100m runner-up Yohan Blake join Bolt in giving the Caribbean stars a powerful line-up in the feature event, but Bolt is set to steal the show.

"I will not relax at all," Bolt said. "I expect strong challenges from even my own teammates in Jamaica and the Caribbean."

Bolt won the 100m and 200m titles in world-record runs, 9.58 seconds in the 100 and 19.19 seconds in the 200, at the last IAAF worlds in 2009 at Berlin and high expectations abound this time around from supporters.

"I never worry about what people want. I'm doing it for myself, and for me the key is the World Championships," Bolt said. "If I lose, I'll feel bad."

Bolt says he is not in the form to break world records this year and is training with a broad goal of peaking next year in London.

"I'm working my way up to the Olympic season, to being fit and ready then," Bolt said. "London is the key."

Bolt won a 200m on July 29 at Stockholm in 20.03 sec into a headwind in his final worlds tune-up, having suffered his only career losses in 100m races at the Swedish capital, to Gay last year and Powell in 2008.

Six days before, Jamaican 4x100m relay man Nesta Carter nearly upset Bolt at Monte Carlo before he rallied to win in a season-best 9.88.

Powell hurled down a gauntlet to Bolt by winning a Diamond League title in Lausanne in 9.78 last month. Only Bolt and Gay have been faster.

"I definitely can't wait to face Usain," Powell said.

Veteran Veronica Campbell-Brown swept the Jamaican 100 and 200 crowns and figures to be a hot contender for world titles in both events. Kerron Stewart was a close second in both races and figures to be on Campbell-Brown's heels, or closer, at Daegu.

I'm just trying to make sure I am physically and mentally prepared to compete when the competition gets ready (at Daegu)," Campbell-Brown said.

Nick Ashmeade and Mario Forsythe will challenge in the men's 200 while Sherone Simpson will run the 200 and rookie Jura Levy will run in the 100.