UFC: Jon Jones removed from fightcard after failing a drugs test
Interim light heavyweight champion Jon Jones will no longer headline UFC 200 after failing a drugs test.
The mixed martial arts fighter, 28, was due to face reigning champion Daniel Cormier, 37, in Las Vegas on Saturday.
As there is not enough time to review the positive sample before Saturday, the bout has been withdrawn.
UFC president Dana White said Jones has the chance "to prove himself innocent" but said it would be "super, super disappointing" news if he is guilty.
In a news conference on Thursday, a tearful Jones apologised to Cormier, saying: "I know this fight means a lot to him. And it means a lot to me."
He said he had taken the same supplements as he did before previous fights and that anything different was not performance-enhancing.
Jones' manager Malki Kawa said they will not name the substance until a B sample is tested by the United States Anti-Doping Authority (Usada)
"Right now, I'm trying to think of everything I've taken," Jones added. "The whole thing sucks. If I do have to serve two years then I will definitely be back."
The heavyweight fight between American Brock Lesnar and New Zealander Mark Hunt will now be the main event.
Lesnar, a star of World Wrestling Entertainment (WWE), is returning to mixed martial arts for the first time since 2011.
Jones tested positive in an out-of-competition sample collection by Usada on 16 June.
White told a separate news conference that he was not aware whether the violation involved a performance-enhancing substance.
The failed drugs test is the latest setback for one of UFC's biggest stars.
Jones also tested positive for traces of cocaine before beating fellow American Cormier in their first meeting in January 2015.
The positive test only came to light after the fight had taken place.
Then in April last year, Jones was stripped of the light heavyweight title he had held since 2011 following a hit-and-run criminal charge.
He was also sentenced to 18 months probation following the incident and suspended from the UFC for several months.
He returned to competition in April this year with a victory over Ovince Saint Preux to take the division's interim belt.
This is the second change to the UFC 200 headline fight.
Irishman Conor McGregor, 27, was due to take on American Nate Diaz, 31, in a rematch of their thrilling contest in March but that fell through.
Middleweight champion Michael Bisping, who has fought at light-heavyweight in the past, said he is ready to take Jones' place if asked by UFC.
The Manchester-based 37-year-old, tweeted: "Ok for the record. I will fight Daniel Cormier. I love him, but if the UFC need me, I'm in!"