Amir Khan's world light-welterweight title rematch against Lamont Peterson on May 19 is in doubt after the show's promoter Richard Schaefer claimed the American has failed a drugs test.
Schaefer said he was informed by the Nevada State Athletic Commission that Peterson had shown abnormally high levels of a banned steroid in a urine sample submitted in March to the Las Vegas-based Voluntary Anti-Doping Agency (VADA).
However, Schaefer said Peterson tested negative in a separate test submitted in April, and the fighter's lawyers were currently making their case to the Nevada authorities in a bid to convince them to grant Peterson a licence.
Schaefer said: "Peterson's legal representatives will be submitting their paperwork to the Nevada Commission today, explaining themselves and why there was a substance (in Peterson's urine).
"Based on those statements from Peterson, the Nevada Commission will rule on if in fact there will be a fight or not.
"I assume we will know by tomorrow (Wednesday). Amir is obviously fully aware of the situation. He is very disappointed but he is going to follow whatever the Nevada Commission is going to rule, and that's where we stand."
Schaefer intimated there may be some hope for the fight because of the subsequent negative test.
However, Schaefer expressed his anger over the late notice of the positive test, with Peterson's camp having been informed of the findings from the 'A' sample on April 13, with the 'B' result following some weeks later.
Schaefer added: "I got a call from the executive director of the Nevada State Commission who informed us he had seen a letter from VADA which confirmed Lamont Peterson has tested positive for a substance consistent with the administration of a steroid.
"A urine specimen was taken from both fighters on March 19 in an unannounced test following the press conference in Los Angeles.
"The first results of Lamont Peterson's sample were reported to VADA by the lab on April 12. VADA informed Peterson on April 13. Why we were not informed is a mystery to me.
"The process of examining the B sample began at the lab on April 30 - why it took from April 13 to April 30 to test the B sample is again a mystery to me - if I'd been informed I would have asked for expedited testing.
"The results of the B sample confirmed the analytical results of the A sample, so basically both the A and B samples show the athlete's specimen is consistent with the administration of an anabolic steroid such as testosterone.
"The letter says follow-up samples taken on April 13 came back negative on May 2, so while the first ones were positive, the second were negative. It is important the Nevada Commission looks at the facts and makes an appropriate ruling."
The news will dismay up to 4,000 Khan supporters who have booked flights and accommodation for the bout, with Peterson set to put the WBA and IBF titles on the line after winning them in controversial fashion in December.