Jones regained the light-heavyweight title belt with a third-round stoppage of Daniel Cormier last month but a sample tested ahead of the bout is being assessed by the US Anti-Doping Agency.
A UFC statement read: “The UFC organization was notified today (Tuesday) that the US Anti-Doping Agency (USADA) has informed Jon Jones of a potential Anti-Doping Policy violation stemming from an in-competition sample collected following his weigh-in on July 28, 2017.
“USADA, the independent administrator of the UFC Anti-Doping Policy, will handle the results management and appropriate adjudication of this case involving Jones, as it relates to the UFC Anti-Doping Policy and future UFC participation.”
UFC president Dana White admitted any lengthy ban for Jones “would probably be the end of his career”, while a statement from Jones’ manager Malki Kawa to several media outlets read: “We are all at a complete loss for words right now.
“Jon, his trainers, his nutritionists and his entire camp have worked tirelessly and meticulously the past 12 months to avoid this exact situation. We are having the samples tested again to determine the validity or source of contamination.
“Jon is crushed by this news and we are doing whatever we can as a team to support him.”
Jones became UFC’s youngest ever world champion when he beat Mauricio Rua in 2011, aged 23. He defended the belt a record eight times, the last against Cormier, but was stripped of his title in 2015 for “violating the Athlete Code of Conduct Policy” after a hit-and-run incident which saw him sentenced to an 18-month probation order.
Two previous rematches fell through – the first after Cormier pulled out citing a foot injury, the second after Jones was suspended for a year following a failed drugs test. He had only returned from that ban earlier in July.