Lamont Peterson has tested positive for banned substances and thrown his rematch with Amir Khan into doubt.

The Unified WBA and IBF junior welterweight champion reportedly tested positive for synthetic testosterone following a random test in March. It is unclear as to why the result only now coming to light.

Jeff Fried, Peterson's attorney, acknowledged the positive test result on Monday night.

"We have tremendous respect for VADA and its mission," Fried said in a statement to ESPN.

"Lamont, [trainer/manager] Barry [Hunter] 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 Athletic Commission.

"Lamont has never had a positive test either before or after this isolated occurrence and we plan to submit the 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."

The rematch between Khan and Peterson was scheduled to take place in Las Vegas on May 19 but has now been put in jeopardy.