Former Ultimate Fighting Championship (UFC) heavyweight champion Brock Lesnar has been flagged by the U.S. Anti-Doping Agency (USADA) for a possible doping violation, the UFC said on Friday.

UFC said it was notified late on Thursday of the potential violation that stemmed from an out-of-competition test on June 28, just days before Lesnar had his first UFC fight in more than four years.

"USADA, the independent administrator of the UFC Anti-Doping Policy, will handle the results management and appropriate adjudication of this case," UFC said in a statement.

"The Nevada State Athletic Commission also retains jurisdiction over this matter as the sample collection was performed in close proximity to Lesnar's bout at UFC 200 in Las Vegas."

Lesnar, 39, competed in UFC 200 on July 9, beating Mark Hunt by unanimous decision in his first appearance in the octagon since December, 2011.

The sample was analysed at the World Anti-Doping Agency-accredited laboratory at the University of California-Los Angeles (UCLA).