Anthony Joshua has been given 48 hours by the WBO to prove his fight with Tyson Fury can go ahead before he is ordered to face Oleksandr Usyk.
The governing body's stance on Wednesday evening is a further blow to the planned undisputed heavyweight championship fight between Joshua and Fury for August 14 in Saudi Arabia.
A court ruled on Monday that WBC champion Fury must take on Deontay Wilder again by September 15, meaning IBF, WBA and WBO titleholder Joshua may instead fight Usyk next.
Usyk was called Joshua's "Plan B" by promoter Eddie Hearn because he is the WBO mandatory challenger.
The WBO had originally agreed to forego Usyk's right to a shot at the belt because of the magnitude of the planned undisputed title fight but Joshua now has two days before he will be ordered to face the undefeated Ukrainian.
"The WBO is extremely happy as their mandatory challenger will be facing their heavyweight champion," Usyk's promoter Alexander Krassyuk told Sky Sports.
"What else can they desire? We are waiting for their approval letter!"
WBO president Paco Valcarcel said on Tuesday night: "I hope boxing fans will enjoy Joshua vs Usyk in the near future."
The targeted date for Joshua vs Usyk is in August at "most likely a big open-air venue in the UK", says Krassyuk.
Usyk had previously been in negotiations to fight Joe Joyce for the WBO interim title while he awaited the winner of Joshua vs Fury.
"Usyk said that he was crossing the English Channel by the time he received my message, and asked which turn he should be making," joked Krassyuk.
"He was happy to hear the news."
Krassyuk had previously reacted to the court ruling Fury to face Wilder: "There is no other reasonable excuse for AJ now. If he is not avoiding Usyk, it is high time to face the mandatory!
"We are now in the position to make the fight happen as soon as possible."
Fury lashes out at Wilder
"What a joke the Bronze Bomber has become," said Fury who drew their first WBC title fight then won the rematch.
"Asked for $20m to move over, joker.
"Looks like I have to [fight him] again."
But Wilder's trainer Malik Scott hit back: "No. Wilder declined and had no interest in step aside money. You are dealing with a whole different type [of mentality] over here. He wants the blood not the step aside money.
"'Retribution is upon us'."
Fury posted a mocking list of Wilder's "excuses" since they last fought: "Costume was too heavy, Fury's gloves were loaded, I had weak legs, my water was spiked, Fury's gloves had no padding, had bicep surgery, disloyal trainer, complications in camp, I broke my arm, fight stopped too early, Andre Dirrell's fault."
Can Joshua vs Fury be salvaged?
Promoter Eddie Hearn considered whether the undisputed title fight may be delayed until December, to first allow Fury to face Wilder and Joshua to meet Usyk.
Asked about the August 14 date in Saudi Arabia for Joshua vs Fury, Hearn said: "Am I hopeful? I don't know.
"We were told the arbitration wouldn't be a problem but obviously it is a problem.
"We have to think on our feet. We have to act accordingly. We still hope the fight can go ahead but it's out of our hands.
"If [Fury's] hands are tied [Joshua] will have to look elsewhere."