Jon Jones has been ruled out of his bout with Daniel Cormier in the main event of UFC 200 after being notified of a potential doping violation.
The organisation's president, Dana White, announced the dramatic change regarding the UFC interim light heavyweight champion just days before the July 9 showpiece, with the heavyweight bout between Brock Lesnar and Mark Hunt the new main event.
Jones tested positive for a banned substance in an out-of-competition sample taken on June 16 by the United States Anti-Doping Agency (USADA), which administers the UFC's anti-drug policy.
With no real time to fully review Jones' case before July 9 it was decided to remove him from the card but there will be a full review, which would include the ability to test his B sample.
Despite being considered the top pound-for-pound fighter in mixed martial arts, Jones has failed drug tests around two of his past three scheduled fights. He tested positive for apparent cocaine use before his first fight with Cormier at UFC 182 in January 2015.
Jones also served a suspension for much of 2015 after his involvement in a hit-and-run accident.
A statement on the UFC website said: "The UFC organisation was notified tonight that the US Anti-Doping Agency (USADA) has informed Jon Jones of a potential Anti-Doping Policy violation stemming from an out-of-competition sample collection on June 16, 2016.
"USADA, the independent administrator of the UFC Anti-Doping Policy, will handle the results management and appropriate adjudication of this case. It is important to note that, under the UFC Anti-Doping Policy, there is a full fair legal review process that is afforded to all athletes before any sanctions are imposed.
"However, because Jones was scheduled to compete against Daniel Cormier this coming Saturday, July 9 in Las Vegas, there is insufficient time for a full review before the scheduled bout and therefore the fight has been removed from the fight card. As a result, the three-round heavyweight bout between Brock Lesnar and Mark Hunt will become the UFC 200 main event.
"Consistent with all previous potential anti-doping violations, additional information or UFC statements will be provided at the appropriate time as the process moves forward."
White told a news conference he was not aware if the violation involved a performance-enhancing substance.
"He's got the chance to prove himself innocent before proven guilty, but if it's true, obviously super, super disappointing," White said. "These type of things are going to happen. It could have been a lot worse. Jones might get two years for this."