Anthony Joshua called Tyson Fury "a fraud" in a furious social media exchange after the news that their heavyweight showdown might be off.
The pair looked set for a highly-anticipated meeting in August, but an arbitration hearing ruled Fury must first face American Deontay Wilder.
"You've let boxing down. You lied to the fans and led them on. Used my name for clout not a fight," tweeted Joshua.
In response, Fury challenged Joshua to a bare-knuckle fight.
The teams behind the two British heavyweights have engaged in lengthy negotiations to get what has been billed as one of the biggest fights in heavyweight boxing for decades over the line.
Joshua's promoter Eddie Hearn was adamant their fight was agreed - and on Sunday, Fury told his Instagram followers he too has signed off on a 14 August bout in Saudi Arabia.
Just 24 hours later, Wilder's team - who had always said he had a contract in place for a third Fury fight - found out their arbitration hearing was successful.
Fury, 32, won the WBC world heavyweight title from Wilder in a rematch in February 2020 after an initial draw.
On Tuesday, Fury claimed that the American, 35, has requested $20m to step aside and allow the Joshua bout to go ahead.
Hearn has since stated the situation is now out of his control and has set Fury's team a deadline of the end of the week to resolve their situation with Wilder.
"If there was an arbitration going on, why announce to the world we are fighting! The fight was signed," Joshua tweeted in a message to Fury.
He also turned down Fury's offer of a "fight this weekend bare knuckles till 1 man quits" for £20m each.
"I'll slap your bald head and you'll do nothing," tweeted Joshua, 31.
Fury then called Joshua a "bottlejob" and claimed: "I'll smoke wilder first then you will get yours aswell (sic)."
Hearn has admitted he is considering alternative plans, notably a contest between Joshua and Ukraine's Oleksandr Usyk, who is mandatory challenger for the WBO title.
Joshua holds the WBO, WBA and IBF belts, meaning a bout with Fury would have been the first at heavyweight for all four world titles.
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