Amir Khan's rematch with Lamont Peterson is in doubt after the American tested positive for a banned substance.
The pair are due to meet for a second time on 19 May at the Mandalay Bay Hotel in Las Vegas, the WBA having ordered a rematch after Peterson defeated Khan in controversial circumstances to win the WBA and IBF light-welterweight titles by split decision on 10 December.
However, the second bout now looks to be in jeopardy after Peterson reportedly gave a urine sample on 19 March which tested positive for synthetic testosterone.
Peterson's publicist Andre Johnson issued a statement to RingTV.com on behalf of the fighter and his manager Barry Hunter, which read: "We have tremendous respect for VADA and its mission. Lamont, Barry and the entire team emphatically support random drug testing in the most comprehensive manner possible.
"We are working expeditiously with a team of pathologists and other medical specialists to confirm the origin of the test result and in full compliance with the rules of the Nevada State Athletic Commission.
"Lamont has never had a positive test either before or after this isolated occurrence, and we plan to submit medical findings by close of business Tuesday reflecting the actual facts in support of Lamont's good faith intentions and the requirements of the commission."
Asif Vali, Khan's business manager, told Sky Sports News: "Obviously last night it was a shock when I received the call to say Lamont had failed a drug test.
"However, the Peterson team seem to think that something's gone wrong during the very first VADA drug test which Lamont took in March. Their explanation is that it's something he took earlier on in the year - before the last fight."
Asked whether the fight was on or off as things stood, he replied: "The fight's on. Obviously the Nevada commission will come back to us with their findings. They'll look at things quite in depth before they make a final decision. That decision's due to come to us by the end of play on Tuesday."
Vali added on Sky Sports News of Khan: "He's very disappointed. Bear in mind he went to the training camp early to get ready for this fight and nobody likes to see a boxer go through preparations for a fight and then find out at the last hurdle that something's gone wrong - certainly when it's not their fault.
"We're 10 days away from the fight. We don't want to make any assumptions in any way, shape or form. The fight is still on at the moment. I know the fans will be disappointed. People waking up will be in shock. People have made plans to go there. We want to do everything possible to ensure the fight's on."
Khan complained to the WBA, IBF and Washington DC commission after losing his light-welterweight titles to Peterson in the American's hometown, Washington, last year.
He highlighted several grievances, chiefly referee Joseph Cooper's deduction of two points, the judges' scoring of the bout and the unexplained presence of 'mystery man' Mustafa Ameen at ringside.