AS A leading author of books on drugs in sport gave warning that “this could go off like a thermonuclear device”, the first hint that athletes caught using a new designer steroid would resort to one of the oldest defences in the book came yesterday from the direction of Kevin Toth, the first athlete to be named in connection with the allegations. “I didn’t know what I was taking” is sure to become the mantra for athletes in the controversy.

Toth’s name was disclosed by a newspaper after the United States Anti-Doping Agency (USADA) announced on Thursday that it had received notification of several leading sportsmen and women, mainly from athletics, who had returned positive A samples for tetrahydrogestrinone (THG), the so-called designer steroid. Terry Madden, the USADA chief executive, described it as apparent “intentional doping of the worst sort”.

The company that was said by the USADA to be the probable supplier of THG is facing a federal investigation over tax evasion and related drugs issues, a case that is due to be heard by a federal grand jury in San Francisco beginning perhaps as early as tomorrow. Leading athletes, including Marion Jones, Tim Montgomery and Kelli White, have been subpoenaed to appear, as well as Barry Bonds and Jason Giambi, two leading baseball players.

Although no test was in place and THG was thought to be undetectable, samples were kept back from the United States Championships in Palo Alto, California, in June and retested after a reliable method was developed at the International Olympic Committee- accredited laboratory in Los Angeles. Some 350 US Championships athletes were retested, as well as 200 more, from track and field and other sports, out of competition.

It is not clear whether British athletes may be among those who have returned positive samples, but it is likely, given the number who train in the US, that some were tested out of competition.

The case before the grand jury is thought likely to reveal more about the scale and detail of the THG issue, which threatens to become the biggest mass doping bust in the history of sport.

Yesterday, USA Today quoted Chuck Yesalis, a drugs books author, as saying: “This could go off like a thermonuclear device. They do not have a broom big enough to sweep it under the carpet this time. I bet NFL (National Football League) security and baseball are trying to build firewalls like crazy now.” The NFL and the IAAF are considering further retesting.

Toth, the US shot champion who this year threw the longest distance in the world since 1990, received products from the Bay Area Laboratory Co-operative (Balco) nutritional supplements laboratory, which is under investigation. “Kevin is saying he did not know what it was,” Howard Jacobs, Toth’s lawyer, said. “If he took it at all, he certainly did not know what it was. He had never heard of tetrahydrogestrinone or THG.”

Under IAAF rules, it matters not whether Toth, or any athlete who may be charged, knew what it was. If their B sample confirms the A, they would be found guilty of a doping offence under the “strict liability” law, which makes competitors responsible for any illegal substance found in their system. For athletes confirmed positive, performances since the date of the test would be annulled, including those at the World Championships in Paris in August.

Victor Conte, the president of Balco, argues that THG is not a steroid. Although it is not specifically on the banned list, it is classed as a related substance and subject, therefore, to the same penalties. “We need to find out more about the test,” Jacobs added. “We do not know if the test is valid or completely bogus. Whether or not this is a steroid remains to be seen.”

Giambi, the New York Yankees first baseman, is the latest name revealed to have been subpoenaed. The list is thought to run to 40 athletes or more. It extends to Bonds, of the San Francisco Giants, Jones, the Olympic 100 and 200 metres champion, Montgomery, the 100 metres world record-holder, and White, who is facing the loss of her sprint gold medals from Paris after testing positive for a stimulant.

Montgomery and Jones are on Conte’s client list, as is Dwain Chambers, Britain’s European 100 metres champion. Inquiries as to whether Chambers may have been subpoenaed have been denied by John Regis, who represents him, while another of his representatives, Mark Brodie, said: “I don’t want to talk about this.” Chambers was said by Remi Korchemny, his coach, to be on holiday.