Anthony Joshua has called Tyson Fury "a fraud" with their undisputed title fight on the brink of collapsing after he was given a 48-hour deadline before being 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.
"The world now sees you for the fraud you are," Joshua posted on social media towards Fury.
"You've let boxing down! You lied to the fans and led them on. Used my name for clout not a fight.
"Bring me any championship fighter who can handle their business correctly."
Fury replied: "You are more full of [rubbish] than [promoter Eddie Hearn]. Your full team knew there was an arbitration going on, it was out of my hands! If I'm a fraud let's fight this weekend bare knuckles until one man quits."
Joshua hit back: "If there was an arbitration going on, why announce to the world we are fighting! The fight was signed!
"I'll slap your bald head and you'll do nothing!"
Fury called Joshua a "bottle job" then added: "I'll smoke Wilder first then you will get yours as well."
Joshua set to fight Usyk
Usyk was called Joshua's "Plan B" by promoter Eddie Hearn because he is the WBO mandatory challenger - that fight is now two days away from being ordered.
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.
"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."
Joshua breaks his silence
Joshua posted on social media shortly after the WBO gave him a 48-hour deadline, joking: "I'm thinking about starting a Go Fund Me page to raise money for Wilder's step aside payment - £20m.
"So I can smoke that fat guy with skinny legs from Manchester.
"These heavyweights need a podcast or something because they do more talking than fighting."
Fury previously said about Wilder: "What a joke the Bronze Bomber has become.
"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."