Britain's Dillian Whyte said he was devastated after an "adverse finding" from a random dope test led to the cancellation of next week's heavyweight fight with former world champion Anthony Joshua.
The two were set to meet at London's O2 Arena on August 12 in a rematch of their 2015 bout, won by Joshua, at the same venue.
Promoters Matchroom said in a statement on Saturday that the Voluntary Anti-Doping Association (VADA) had informed them that Whyte "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," they added.
Whyte, who lost a title challenge to WBC world champion Tyson Fury in April 2022, reacted to the news on messaging platform X, formerly known as Twitter.
"I can confirm without a shadow of doubt that I have not taken the reported substance, in this camp or at any point in my life," he said.
"This is not the first time that I have been reported as having an adverse finding for a substance which I have not taken, and as I did last time I will again prove that I am completely innocent."
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.