Anthony Joshua will fight Robert Helenius on Saturday, with the Finn coming in as a replacement for original opponent Dillian Whyte.
Joshua was due to box old rival Whyte at the O2 Arena this weekend.
But that heavyweight bout was cancelled after a random anti-doping test on Whyte returned "adverse analytical findings".
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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.
Helenius boxed last weekend, beating Mika Mielonen by stoppage on Saturday, and will step in at short notice to take on former unified champion Joshua.
"This wasn't in the script," Joshua said. "I respect Helenius and may I say, I respect any male or female who steps into the ring. I am laser focused on the win. I can make steps forward to bigger and better things but the road map has a check point, Saturday night. May the best man win."
The 39-year-old Finn was knocked out by Deontay Wilder in one round last year, though prior to that he had secured respectable back-to-back wins over Adam Kownacki. He has a 32-4 (21) record and three of his defeats have come inside the distance.
"I am excited about fighting Anthony Joshua on August 12," Helenius said. "I am a true Viking that is willing to face any challenge at a moment's notice. This is not an opportunity I was going to let slip away. I plan to make the most of it."
Although Whyte will not be boxing, he is protesting his innocence. He wrote: "I am shocked and devastated to learn of a report by VADA of adverse findings relating to me.
"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."