Anthony Joshua fired a broadside at British rival Tyson Fury, calling the 'Gypsy King' a "fraud" and accused him of "letting boxing down" after their world heavyweight unification title fight was thrown into doubt.
Joshua and Fury, who between them own all four heavyweight titles, were set for the first of two blockbuster fights on August 14 before a US judge this week ordered that Fury must fight American challenger Deontay Wilder before September 15.
On Tuesday, Fury claimed that Wilder had requested $20 million to "move over" and allow the Joshua bout to go ahead.
Joshua questioned why Fury, 32, had announced they would fight when arbitration proceedings were still going on.
"Tyson Fury the world now sees you for the fraud you are. You've let boxing down. You lied to the fans and led them on. Used my name for clout not a fight," Joshua tweeted.
"Bring me any championship fighter who can handle their business correctly."
Fury (30-0-1) responded by saying Joshua's team knew arbitration was going on and the matter was "out of my hands". He also challenged Joshua to a bare-knuckle fight.
"I'll slap your bald head and you'll do nothing," Joshua, 31, tweeted in response.
Joshua's promoter Eddie Hearn is considering alternative plans for his fighter, including a bout with Ukraine's Oleksandr Usyk - the mandatory challenger for the WBO belt.
"We want to be in a position by the end of this week to know are we fighting Tyson Fury or are we moving forward with another option," Hearn, the head of Matchroom Boxing, said.
Joshua (24-1) was set to put his IBF, WBO and WBA titles on the line against WBC belt-holder Fury on August 14 in Saudi Arabia in the first of two fights announced by the pair earlier this year.
Fury, undefeated in 31 professional bouts, became a two-time world champion when he scored a sensational seventh-round stoppage over Wilder in Las Vegas 15 months ago. The first fight, in December 2018, had ended in a draw.