The news which has rocked the athletics world:

BBC article:

The ramifications of Justin Gatlin failing a drugs test are extensive and serious for both the American and his sport.

The world and Olympic 100m champion - and joint holder of the world record - was found to have elevated levels of the hormone testosterone after a meeting in April, it has been revealed.

Both his primary and back-up samples have tested positive, meaning Gatlin now faces a fight to clear his name.

What is testosterone and what does it do?

Testosterone is a hormone that occurs naturally in both men and women, and can also be manufactured articificially and administered externally.

It helps build muscles, increasing speed, stamina and aggression and helping recovery.

It is listed with anabolic steroids on the World Anti-Doping Agency's list of prohibited substances.

A urine test measures its ratio with epitestosterone, a hormone produced along with testosterone but without its effects. The ratio in most people is 1:1. The ratio accepted in athletes' drugs tests is up to 4:1. Gatlin's levels have not yet been published.

What does it mean for Gatlin?

Gatlin, who denies knowingly taking drugs, is facing a lifetime ban from athletics unless he can prove the positive test came about through exceptional circumstances.

He would almost certainly also be stripped of the world record he shares with Jamaica's Asafa Powell. Gatlin equalled Powell's mark of 9.77secs in Doha in May, the month after his positive test.

Athletics operates a strict liability policy, which dictates that athletes are responsible for any substance found in their body.

A first offence would normally carry a ban of two years, but Gatlin failed a previous drugs test when he tested positive for amphetamines in 2001.

His suspension was lifted early after he persuaded athletics' governing body the IAAF that the test failure had been caused by medication he was taking.

But the IAAF made it clear at the time that he would be banned for life if he failed another drugs test, and it reiterated that stance on Sunday following the news of Gatlin's positive test.

How credible is the claim of Gatlin's coach that the sprinter was the victim of sabotage?

Athletics legend Michael Johnson has dismissed this suggestion by Trevor Graham as being "an old excuse" from a man with "no credibility".

Gatlin's lawyer Cameron Myler said Graham's comments were "not made with the knowledge or authorisation of either Justin or us", adding: "At this point we are trying to figure out what was the cause of the positive test."

Gatlin is the latest in a series of athletes trained by Graham to find themselves mired in a doping scandal.

Graham has worked with six other world champions who have tested positive - the shot putter CJ Hunter, 400m runners Alvin Harrison, Calvin Harrison, Antonio Pettigrew and Jerome Young, and the 200m athlete Michelle Collins.

He also coached former 100m world record holder Tim Montgomery, who despite never failing a drugs test was banned as part of the doping scandal arising from the California-based nutritional supplement company Balco.

Graham is a central figure in the Balco scandal. He was the whistle-blower who started it by supplying the authorities with a syringe containing the newly-created steroid tetrahydrogestrinone (THG).

Will Gatlin actually be banned?

Governing body US Athletics has come under fire in the past from world anti-doping chief Dick Pound for not doing enough to fight doping in the country.

But there has been a series of high-profile doping bans in America in recent years, so Gatlin's chances of escaping punishment do not look good.

Is testosterone easy to detect?

An effective test for elevated testosterone levels has existed for many years. There are complications arising from the fact that it is naturally produced in the body, so drug testers have to be satisfied the levels were not natural.

In Gatlin's case, it has been established that the testosterone found in his body was administered artificially.

Gatlin tested positive for "testosterone or its precursors", which means that the substance responsible for the positive test may have been artificial - testosterone precursors can be found in sports supplements.

Athletes have been warned about these supplements because the industry is not fully regulated and they may be contaminated with prohibited substances.

What happens next?

Gatlin faces a hearing before the US Anti-Doping Agency, possibly early next week.

If he is banned, as seems likely, he will probably appeal, and the saga could rumble on for months or even years.

How big a blow is this for athletics?

Michael Johnson, holder of the 200m and 400m world records despite retiring in 2000, says "doping is still a huge problem for athletics".

Just as cycling finds its reputation in the gutter following Tour de France winner Floyd Landis testing positive for testosterone, so it is with Gatlin and athletics.

His rivalry with Powell was the sport's most compelling storyline, and was building towards one of the most eagerly-anticipated showdowns in years. Now that race will probably never happen.

The men who run the 100m are the standard-bearers for the sport. It is the event with the biggest and most straightforward public appeal - it establishes who is the fastest human being on the planet, a hugely evocative concept.

It is hard to think of anything more damaging than the man who jointly holds that title - and who is also the reigning champion in the sport's two most high-profile championships - being exposed as a drugs cheat.

Even if Gatlin eventually clears his name, his reputation, as Johnson has said, will forever be tarnished.

And coming on the heels of the exposure of a number of other world-famous names, this is a revelation from which the sport's reputation will take a long time to recover.