Usain Bolt, the world's fastest man, believes it will take time for people to believe that his performances are possible without doping.

The 23-year-old Jamaican, who won three Olympic titles in world record times in Beijing last year, clocked the fastest 100metres ever recorded in Berlin in August, winning the World Championship title in 9.58 seconds - an astonishing 0.11secs faster than his previous world best.

People have watched in disbelief since Bolt set the world record in only his fifth 100m race at the Reebok Grand Prix in New York on June 1, 2008.
And he has once again reiterated that he does not use performance-enhancing substances and believes he will confound the doubters by continuing to run quickly.

"I'll continue running fast, I am clean without a doubt," said Bolt, who has four of the 10 quickest 100m times in history and five of the top-10 200m times.

"As soon as you start running fast there's going to be a problem.

"It's going to take a couple of years for people to start believing that you're actually clean.

"If you continue running fast and doing it clean then over a period of time all the tainting will go away.

"People will continue to say it until that period of time."

He added: "I've been saying this all season. People always say things, the sport has been through a lot, but it's changing, definitely.

"The IAAF is doing a very good job of cleaning up the sport."

Bolt, who was speaking in London at the launch of The Zeitz Foundation and The Long Run - a new global initiative by Jochen Zeitz, chairman and chief executive of Puma - is also confident of defending his Olympic 100m, 200m and 4x100m relay titles in London, believing the capital's Jamaican community can spur him on to more success.

"The Olympics is going to be a very big thing for me because I did extraordinary things in Beijing," he added.

"I'm definitely coming here to defend my titles and I'm looking forward to it because there are a lot of Jamaicans here so I know it's going to be a good Games.

"I'm just going to come here and do my best as always."