The long-awaited rematch between Lamont Peterson and British boxer Amir Khan has been thrown into doubt after the American failed a drug's test, a report said Tuesday.

Peterson and Khan's May 19 bout in Las Vegas is now under a cloud after the WBA and IBF light-welterweight champion tested positive for synthetic testosterone on March 19, reports said.

Both Khan and Peterson had agreed to random testing by the Voluntary Anti-Doping Association (VADA) ahead of the fight, which comes six months after their first meeting in December last year.

Peterson's publicist Andre Johnson said in a statement to RingTV.com that the boxer's camp was mystified by the test results.

"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," he said.

"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."

Peterson's split-decision victory over Khan last year was mired in controversy, with the British fighter claiming a "home-town decision" had robbed him of victory.

Meanwhile, Khan's business manager Asif Vali 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."