Eddie Hearn, promoter for IBF, IBO, WBA, WBO heavyweight champion Anthony Joshua, says WBC champion Deontay Wilder is their only target for April 13 at Wembley in London.
Earlier this month, Wilder fought to a controversial twelve round split draw with Tyson Fury at Staples Center in Los Angeles, California.
Now the main focus for Wilder is certainly a highly anticipated rematch with Fury - likely to take place in the first half of 2019.
Joshua retained his titles back in September with a knockout victory over WBA mandatory challenger Alexander Povetkin.
But if Wilder is not going to take part in the contest, then Hearn will focus on other options.
The frontrunner would become the winner of next Saturday's rematch between Dereck Chisora and Dillian Whyte. Heavyweight contender Jarrell Miller is another option, and so is unified cruiserweight champion Oleksandr Usyk.
Joshua will be ringside next weekend, at the O2 in London, to scout the Whyte-Chisora rematch.
The unification between Joshua and Wilder has been discussed in the past, with the two sides being unable to reach an agreement.
"He (Joshua) just came out and told the truth: I want Deontay Wilder on April 13 - that's my fight date at Wembley," Hearn exclusively told Sky Sports.
"Why not? If you want to be undisputed, now's the time, so he's made it clear, to the public, to me, that's what he wants. If Wilder fights Fury, he [Joshua] has to fight the winner of Whyte-Chisora or Jarrell Miller, or [Oleksandr] Usyk, or one of these guys.
"But there is absolutely no poll required for Anthony Joshua. It's just Deontay Wilder, just the undisputed heavyweight championship of the world. Right now, our focus is on Deontay Wilder on April 13, at Wembley. If he doesn't want to fight Anthony Joshua, there's nothing we can do, but that's what we want."