Dillian Whyte's "adverse analytical findings" from a random anti-doping test have prompted the cancellation of his heavyweight clash with Anthony Joshua.
The Voluntary Anti-Doping Association said a "random anti-doping protocol" undertaken by Whyte had returned the adverse findings, leading to the all-British grudge match being called off.
Joshua's rematch with Whyte had been due to take place at London's O2 Arena on August 12.
Fight promoter Matchroom said in a statement: "Today, the Voluntary Anti-Doping Association (VADA) informed Matchroom, the Association of Boxing Commissions and the British Boxing Board of Control that Dillian Whyte had returned adverse analytical findings as part of a random anti-doping protocol.
"In light of this news, the fight will be cancelled and a full investigation will be conducted."
Whyte lost his unbeaten 16-fight record when he was stopped by Joshua at the O2 Arena in 2015 when contesting for the vacant British heavyweight title.
The rematch was announced in July after talks over Joshua taking on Tyson Fury broke down.
Former world champion Joshua had also been linked with a fight against Deontay Wilder in Saudi Arabia.
Joshua's win over Whyte almost eight years ago avenged a defeat to his Jamaica-born rival when they had met as amateurs.
Whyte recovered from his defeat to Joshua by winning his next 11 fights and after losing to Alexander Povetkin in 2020 he gained revenge by stopping the Russian in a rematch.
Defeat to defending WBC champion Tyson Fury at Wembley in 2022 was followed by a points win over Jermaine Franklin the following November.
