Former world heavyweight champion Lennox Lewis says he is "disappointed" after Anthony Joshua called him a "clown" who he does not respect.
Following Joshua's defeat by Andy Ruiz Jr on 1 June, Lewis questioned whether the Briton was fully prepared.
He also tweeted that Joshua's trainer Rob McCracken may not offer all that is needed to "rule" at heavyweight.
"Lennox is a clown. I don't respect Lennox," Joshua, 29, told Sky. "Me and Lennox are cut from a different cloth."
Joshua said his own legacy will be to "enjoy" and "appreciate" the next generation of fighters coming through, adding: "Lennox isn't like that."
Lewis responds as Ruiz Jr wades in
Andy Ruiz Jr responded to a tweet asking who would win in a fight between Joshua and Lewis
Lewis, 53, claimed the mantle of 'undisputed champion' at heavyweight in 1999, when he held the division's three recognised belts - IBF, WBA and WBC.
The strengthened reputation of the WBO title has led to present-day fighters seeking four major belts to claim the same 'undisputed' description.
Joshua was chasing that honour but could not agree a fight with WBC champion Deontay Wilder and ultimately went on to lose his IBF, WBA and WBO titles to Ruiz in New York.
In his interview with Sky Sports, he said he was no longer "worrying about undisputed".
Lewis tweeted: "He's right! We are cut from a different cloth. Undisputed wasn't something I 'worried' about. It was something I went after until it was accomplished!
"Disappointed in AJ's words but I understand that this 'jealousy' narrative was only fabricated after I criticised them for Wilder negotiations. All of the sudden I'm a hater.
"Ask who benefits from such a simple-minded narrative? Not AJ. Not me."
The debate between the two former world champions has prompted plenty of reaction on Twitter - and when one account asked who would win in a fight between the pair, unified world heavyweight champion Ruiz replied: "Lennox all day."
Joshua and Ruiz are at a stand-off in negotiations for a proposed rematch in Saudi Arabia on 7 December.
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