Yesterday Olympic favourite Dwain Chambers became the first Briton to test positive for a previously undetectable steriod.

Nick Morrison looks at what could be the biggest doping scandal in the history of sport.

JUST two days after witnessing one of the most thrilling encounters in athletics, the world awoke to a very different kind of buzz. Barely had the excitement of watching the 100m final at the 1988 Seoul Olympics died down, than we were confronted with the shock of seeing the winner exposed as a drugs cheat.

With the acclaim for becoming the fastest man in history still ringing in his ears, Ben Johnson had been stripped of his medals and was flying home in disgrace. Two years of rumour and innuendo, previously put down to the jealousy of the defeated, had finally been proved to be based on fact. Johnson had tested positive for steroids: a remarkable transformation from an also-ran in 1984 to world record holder four years later was found to have been artificially induced.

But the furore over Johnson could pale beside the doping scandal now brewing, which threatens to engulf world athletics, and even draw other sports into its maw. Olympic titles and world records could be discredited, and next year's games in Athens diminished, as many of the most famous names in track and field are found to have cheated their way to the top.

Up to 20 American athletes tested positive for the steroid tetrahydrogestrinone (THG) at this year's US championships. THG is a banned performance-enhancing substitute, which had been subtly altered by chemists to make it undetectable by standard drug tests.

Anabolic steroids help build muscle mass, increasing strength and power, and enable competitors to recover quickly from training, making them particularly popular for athletes in events where speed and strength are at a premium.

But earlier this year, an anonymous athletics coach gave the US Anti-Doping Agency (USADA) a syringe containing THG, claiming several top athletes were using the drug. The USADA used the syringe to develop a test, and then re-examined urine samples taken at the US championships, producing the list of up to 20 competitors who tested positive.

So far, the only name to emerge is shot put champion Kevin Toth, but reports are circulating that the names of Olympic champions and world record holders are on the list. And the implications spread far beyond those 20.

According to USADA's Terry Madden: "What we have uncovered appears to be intentional doping of the worst sort. This is a conspiracy involving chemists, coaches and certain athletes using what they developed to be undetectable designer steroids."

The USADA has traced the source of THG back to Balco, the Bay Area Laboratory Co-operative, which makes nutritional supplements. Among Balco's clients are athletes Marion Jones and Tim Montgomery, tennis players Ivan Lendl and Jim Courier, as well as baseball and American football stars. Balco also claims to have helped Dwain Chambers, the European 100m champion and one of the favourites for gold in Athens.

Yesterday, it emerged that Chambers had tested positive for THG, the first Briton to be caught up in the scandal. A second, or B, test is yet to take place, and Chambers' solicitor said yesterday that the sprinter would fight any accusation that he took the steroid deliberately, but if he is found guilty he faces a minimum two-year ban from international competition, and a life ban from Olympic competition.

And the scandal could take on a new dimension today, when the first of 40 sports stars appears before a federal grand jury investigating Balco for alleged tax evasion. Those scheduled to answer questions include men's 100m world record holder Montgomery and five times Olympic gold medallist Jones, as well as San Francisco Giants baseball star Billy Bonds, and Kelli White, the world 100m and 200m champion. White already faces losing her gold medals after testing positive for the stimulant modafinil, which she claims she took for narcolepsy, or sleeping sickness.

White could also lose her £72,000 prize money, and it is the rewards available in athletics today, not just in prize money but more significantly in sponsorship, which many believe has led more competitors to seek unfair advantage.

"If you become Olympic champion or world champion today, it means that you become a millionaire. Obviously, the motivations and temptations are understandable - but it's very disappointing," says Lyn Davies, president of UK Athletics and a former Olympic long jump gold medallist.

He says UK Athletics does everything it can to warn athletes of the dangers of taking drugs, and is vigilant in trying to uncover the cheats, but John Brewer, head of human performance at the National Sports Centre in Lilleshall, says it is very difficult to stay ahead of the drugs cheats.

"The one thing you will always find with elite athletes is that they will look for something to give them an edge. Hopefully, in the majority of cases that will be legal, different training or a different diet. But people will seize on something, even if there is no proven research that suggests it actually has a genuine benefit.

"It's very, very difficult to stay ahead of the cheats. All the time, athletes and the shady medical characters who are advising them are always looking to stay one step ahead of the field. They are always looking for new ways of avoiding detection."

Paula Radcliffe, who has been an outspoken anti-drugs campaigner, says it is difficult to discover the extent of the problem. "The testers are closing the gap, but at the same time they are always one step behind. We don't know how big a problem it is, because we aren't doing the tests."

Previously, the attitude of US sports chiefs has been one of the biggest obstacles to uncovering drugs cheats. By refusing to come up with its quota for funding, the US has effectively blocked research into developing new tests, allowing the determined cheat to evade detection.

The US has also been less than helpful in the campaign to rid sport of drugs by its behaviour towards athletes who have tested positive. After Kelli White tested positive for modafinil following the World Championships in Paris, it emerged she had tested positive for the same drug at the US Championships two months earlier. Several more positives for the stimulant have been detected in samples from other athletes, suggesting either it is being used to enhance performance, or sleeping sickness is widespread among American athletes.

After Ben Johnson tested positive, he confessed to using steroids and served a two-year ban, later receiving a life ban after another positive test. But when he accused other athletes of also cheating, no one wanted to listen. It was only earlier this year, 15 years on from Seoul, when it emerged that Carl Lewis, who had been promoted from silver to gold following Johnson's disgrace, had tested positive for stimulants at the US Olympic trials that year. After pleading that their use was inadvertent, Lewis was allowed to compete and news of his failed tests was covered up.

As long as there is money to be made - for the sportsmen who taste glory and the chemists who can sell their products - and governing bodies willing to collude for the sake of national honour, it may be that professional sport is forever tainted by suspicion. And whatever the outcome of the THG scandal, however many champions are stripped of their medals and records are expunged from the record books, perhaps the real result is the loss of whatever faith still remains in professional athletics. Even if the cheats are unmasked, will we ever be able to trust the Olympics again?