Dillian Whyte's fight against Anthony Joshua has been cancelled, after a random anti-doping test on Whyte returned "adverse analytical findings".
The Voluntary Anti-Doping Association said it had informed the Association of Boxing Commissions and the British Boxing Board of Control that a "random anti-doping protocol" undertaken by Whyte had returned the adverse findings.
Whyte took to social media to deny taking the reported substance and vowed to prove that he was "completely innocent".
Matchroom said in a statement: "In light of this news, the fight has been cancelled, and a full investigation will be conducted." Sky Sports News have contacted Whyte's team for comment.
The fight had been due to take place at London's O2 Arena on Saturday, August 12. Joseph Parker's promoter David Higgins has said his fighter would step in as a replacement for a rematch with Joshua.
The pair met in Cardiff 2018 when Joshua inflicted the first loss of Parker's heavyweight career, beating the New Zealand native by unanimous decision.
- Savannah Marshall mandatory Shadasia Green wants 'biggest' fight in Newcastle
- Chris Eubank Jr brings in Terence Crawford's trainer for Liam Smith rematch
- Stream Sky Sports boxing and more with NOW for £26 a month for 12 months
"Joseph Parker is already booked to fly back to the UK this weekend - he is fit, ready, willing and able to fight Anthony Joshua at O2," Higgins said in a statement.
"The only road blocks would be Joshua not wanting the fight, or Joshua offering Parker stupidly short money to avoid the fight. I've called Eddie Hearn but I'm hitting voicemail."
Whyte: I am completely innocent
Reacting to the cancellation of his fight with Joshua, Whyte wrote: "I am shocked and devastated to learn of a report by VADA of adverse findings relating to me. I only learned of it this morning and am still reacting to it.
"I have also just seen that the fight is being cancelled without having any chance to demonstrate my innocence before the decision was taken.
"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.
"I am completely innocent and ask to be given the time to go through the process of proving this without anybody jumping to conclusions or a trial by media.
"I insisted on 24/7 VADA testing for this fight, as I have done voluntarily and at my own expense for all of my fights for many, many years.
"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.
"In the meantime, all I can do is express my extreme disappointment to boxing fans, who will miss out on what was sure to be a great event."
The cancellation of Joshua's fight with Whyte is the latest twist in a long-running rivalry between the pair. Whyte defeated Joshua in the amateur ranks before he went on to win an Olympic gold medal at London 2012.
As professionals, they boxed in a wild 2015 British heavyweight title bout, which Joshua won by seventh-round stoppage.
"Yeah, it would be [dream scenario Whyte, Wilder, and then Fury]. But dreams are fake. This is reality," Joshua said.
When asked if he believed he had it in him to beat all three men, Joshua replied: "Yes."
The winner of Joshua and Whyte's now cancelled fight had been linked with a potential clash with Deontay Wilder, eyed to take place in the Middle East later this year.