Dwain Chambers has been suspended pending a disciplinary hearing after his B sample tested positive for banned steroid THG.
The 25-year-old faces a suspension of up to two years which could deny him the chance to compete in the Athens Olympics next year.
Under British Olympic Association rules, he would also be banned from all subsequent Olympics.
Chambers' B sample was tested at a laboratory in Los Angeles, prompting a statement from UK Athletics that read: "The B sample
analysis has been conducted and confirms the finding in the A sample.
"Dwain Chambers has now been suspended by the IAAF pending the result of a
hearing before an independent Disciplinary Committee."
Chambers has protested his innocence, his legal advisors stressing that contrary to earlier reports, there has never been an
admission by their client that he took THG or any other banned substance.
The athlete has also claimed he was categorically
assured that the only supplements he had been given were those which complied
with IAAF anti-doping rules.
Chambers, who is the European 100m champion, first tested positive at a training camp in Germany on 1 August.
UK Athletics chief David Moorcroft revealed that Chambers has until 8 December to request a hearing.
In a statement, Chambers' lawyer Graham Shear said: "I believe the results of the B test do not provide any conclusion to this matter whatsoever."
He added: "This is just the beginning of a lengthy process.
"Until satisfactory explanations and answers for these questions can be provided, nobody is in a position to make judgement on Dwain Chambers' alleged culpability."
John Regis, a director of Stellar Athletics Club which manages Chambers, released a statement supporting his client.
Regis also called for a system of accreditation for coaches, nutritionists and supplement manufacturers to be set up.