Ali fought Zora Folley in 1967 but then missed several years due to his refusal to fight in the Vietnam War, before making a winning return in 1970 and going on to become world champion once more.

McGregor meanwhile has recovered from surgery on a broken leg, and starred in the film Road House, and will also have been away from combat for three years.

That ends at UFC 303 in June (live on ESPN in the US) when he takes on Michael Chandler, and there is plenty on the line. A defeat would likely put the pressure on the Irish former world champion to retire from mixed martial arts.

Speaking to BoyleSports - who provide the latest boxing betting -  Bellew praised McGregor, but gave Ali the edge when it came to rating comebacks.

He said: “Muhammad Ali coming out of hiatus is the greatest comeback of all time, after everything that happened with the war, after he refused to go to Vietnam, that's the greatest comeback of all time. 

“That's not up for debate, Muhammad Ali is the greatest sportsman and people who have ever lived, but I do love Conor McGregor. 

“He's a star and I've had the pleasure of being in his company a few times. He's so lively, he's the life and soul. He should be so proud of himself to get where he's got from where he came from.”

Of the fight with Chandler, Bellew said: “It's a tough fight, I don't know who wins. I’ll have to see what's left of him first because he lives an extravagant life, let's not kid ourselves. 

“It's hard to get out of bed when you're wearing silk pyjamas on a Lamborghini yacht. But I give him the ultimate respect.”

Live on DAZN PPV: Fury vs. Usyk on May 18. Click here for details.