Anthony Joshua has labelled Lennox Lewis "a clown" as their long-running feud escalated.
Lewis, the last undisputed heavyweight world champion, has long been a critic of Joshua who lost his three titles to Andy Ruiz Jr in June.
Lewis has previously called on Joshua to stop "chasing the money" and to ditch trainer Rob McCracken following his first professional defeat.
And Joshua has now hit back in a documentary screened on Sky Sports.
"Lennox is a clown. I don't respect Lennox," he said on AJ: The Untold Truth.
When it was put to him that Lewis left an impressive legacy, Joshua retorted: "So? So am I.
"Me and Lennox are not the same. My legacy is to sit back and enjoy the younger generation coming up, and not to be involved. Just to appreciate what it takes to get there.
"Lennox isn't like that; me and Lennox are cut from a different cloth."
Joshua is due to take on Ruiz again on December 7 in Saudi Arabia - but the new champion is refusing to travel to the desert.
Promoter Eddie Hearn officially announced the date and venue on Monday but Ruiz has since said he wants the fight to be held in America.
"I know everybody is talking about the fight and all that but we got real news coming soon.
"Of course [I'm taking the rematch] but it's going to be on our terms; we're going to bring it back to the United States."
While answering fans' questions on Instagram , he accused Joshua of being "scared" and added: "Why do you think he's over there trying to make the fight in Saudi Arabia?
"Like I said, I don't have any protection over there, so if the United States of America wants to... never mind, I can't say nothing.
"The fight is going to happen soon."
Hearn, however, responded by saying Ruiz could face legal action if he refuses to accept the terms.
