Conor McGregor said Tyson Fury "bottled it" for not stepping in when Billy Joe Saunders' dad was shoved by security at the Canelo Alvarez fight last year.
After Saunders was stopped by Alvarez in their middleweight world title clash last May, the Brit's father attempted to jump over the barrier and console his son.
He was then forcibly shoved backwards into the railings by a security guard, with members of Saunders' team quickly jumping over the barrier to help him as a scuffle unfolded.
But the 6'9" heavyweight champion Fury opted to stay out of the trouble and watch on from behind the railing, with his actions being slammed by McGregor.
Replying to a tweet in which Fury praised McGregor's longtime UFC rival Khabib Nurmagomedov, the Irishman mocked Fury for not defending Saunders' father.
"Yup the Joyces. Big Joe! U bottled it with Billy joe Da, what were you doing there ? ULeft him, Versace t*** U.Done. Nothing, McGregor wrote to Fury on Twitter.
"Ya’s did it right ye right hahahahaaj. Even when I’m wrong i’m right. Who did it right ? The Mac daddy tonight, in house. Miami d12! Bahamas."
McGregor wasn't the only fighter to criticise Fury for not backing up Saunders' father in the brawl, as his domestic rival Dillian Whyte also labelled him a "coward".
"[ Tyson Fury ] is a coward how can you stand by and watch Billy Joe Saunders' father get treated like this and you didn’t jump the barrier and at least try to stop them man handling him. You are a coward all mouth," he wrote to Fury.
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Saunders suffered a broken eye socket in his loss to Alvarez and contemplated retirement after the fight, but recently admitted he misses the sport.
Fury advised his longtime friend Saunders to not retire after the loss and said it would "haunt him forever" if he called it quits.
'The Gypsy King' is set to fight in the UK for the first time since 2018 as he defends his WBC title against interim titleholder Whyte on April 23 at Wembley Stadium.
Whyte has been waiting for his shot for over 1,000 days, but contracts for the fight are yet to be signed.
And Fury has slammed his rival for refusing to comment on the fight since it was announced, stating: "Now the fight has been put on his toes, he’s gone absolutely silent and who’s the coward now?
“I think he’s trying some mind games, but he’s messing with the king of mind games."
