LAS VEGAS (AP) — UFC interim light heavyweight champion Jon Jones was notified of a potential doping violation on Wednesday night, ruling him out of his bout with Daniel Cormier in the main event of UFC 200.

A grim-faced UFC President Dana White announced the dramatic change three days before the mixed martial arts promotion's landmark show.

"He's got the chance to prove himself innocent before being called guilty," White said. "But if it's true, it's obviously super disappointing."

Jones tested positive for an unspecified banned substance in an out-of-competition sample taken June 16 by USADA, which administers the promotion's anti-drug policy. While Jones is considered the top pound-for-pound fighter in MMA, he has apparently failed drug tests around two of his past three scheduled fights.

Brock Lesnar's heavyweight bout with Mark Hunt is UFC 200's new main event at T-Mobile Arena in Las Vegas. Lesnar's much-anticipated return from a 4 1/2-year MMA absence was previously the penultimate bout on the star-studded pay-per-view card assembled by the UFC for its biggest event of the summer.

White said he doesn't know what substance was detected in Jones' test, but acknowledged that this violation could lead to a multiyear suspension for one of the UFC's biggest stars.

After Jones beat Cormier by clear decision in their first meeting in January 2015, the UFC announced that Jones had tested positive for apparent cocaine use before the fight. The detected cocaine metabolite was not banned for out-of-competition use by the Nevada Athletic Commission, which claimed it couldn't stop him from fighting.

But Jones' behavior outside the cage repeatedly has hampered his meteoric rise inside his sport. Shortly after his first positive drug test, he was suspended for several months in 2015 due to his involvement in a hit-and-run accident in Albuquerque, New Mexico, where the upstate New York native lives and trains.