Usain Bolt's training partner Warren Weir insisted sprinting was not tainted by doping as the athletics world governing body emphasised its commitment to the fight against drug cheats.

The International Association of Athletics Federations (IAAF) yesterday approved a return to four-year bans for "serious doping offences" from the start of 2015.

IAAF president Lamine Diack said: "This is a fight that will never end. We will always fight against it."

The issue has overshadowed the build-up to the World Championships, which get under way at Moscow's Luzhniki Stadium tomorrow. Two of the world's leading sprinters, Tyson Gay and Asafa Powell, will not be there after failing drug tests.

Bolt, always the star of the show, is in Russia, though, and his Jamaican team-mate Weir said fans could still trust what they saw on the track.

"You can still believe there are good athletes out there," said the London 2012 200 metres bronze medallist.

"I myself am one of the clean ones. You can't bash all for some.

"People want to see people running fast and clean and it's always good to show them there are still clean ones out there."

Great Britain's notorious former drug cheat Dwain Chambers is also in Moscow, eyeing a place in the 100m final, and admitted he was stunned that athletes had not learned from his sorry tale.

The 35-year-old was shunned by the sport after testing positive for the steroid THG 10 years ago.

"It's sad to see that people I look up to have fallen in the same trap that I once did," he said.

"Unfortunately they haven't learned from my experience, it's disappointing to see when this happens.

"I don't know what motivates people to do these things.

"They should basically be able to look at what I went through. It's been a tough road for me and that itself should be enough of a deterrent for some of these youngsters or athletes.

"They've seen what I've gone through, they've seen what the relevant punishments are. It's something that I wouldn't want to encourage anybody to do, because the penalties are so harsh that they wouldn't be able to come back into the sport with any credibility."

Paula Radcliffe, the marathon world record holder, called for nations guilty of institutionalised doping to be banned from participating in the World Championships.

Russia and in particular Turkey have been plagued by a spate of positive tests, with the latter's athletics federation this week announcing it had suspended 31 track and field athletes for two years for drug offences.

The 39-year-old also said young athletes being forced to cheat was akin to "physical and sexual abuse".