Floyd Mayweather knows his way in and out of the ring and has now set his sights as a boxing promoter... to the bout of the Justin Bieber versus Tom Cruise. Picture: Bang Showbiz

Former boxing champion Floyd Mayweather has offered to promote Justin Bieber and Tom Cruise's fight if it ever actually happens.

Bieber contacted Dana White on Twitter last month to ask if he'd be willing to stage a bout between himself and the Mission: Impossible actor, and though he later claimed his post was a "joke", the UFC boss revealed "real people" had been in touch with him to explore the possibility of staging the fight.

Now the 42-year-old former boxing pro, who once trained Bieber and invited him down to the ring on many occasions to watch his matches, before they allegedly fell out in 2017, has admitted he would only be willing to help either of them if they give him the promoter role.

He told TMZ: "If they bring me in as a promoter, then I can talk about it. But until then, I gotta stay in my lane. Gotta stay in my business."

White had admitted he didn't initially take Bieber's tweet seriously because he's so used to seeing people in the public eye challenge one another to get in the ring.

He told the gossip website recently: "Let me tell you something interesting, obviously this came out, lots of people talk about fighting in the UFC, NFL players or celebrities or whatever, it happens a lot. "I saw it but didn't pay much attention to it.

"I'm not gonna say any names but I got a phone call from a couple of real guys who say they really do wanna do this fight and they believe Tom Cruise would do the fight.

"I told them that if that's true – and everybody involved really wants to do it – we can talk."

And he admitted he'd be willing to do whatever he can to get the 25-year-old singer to face off against the 56-year-old action star.

He said: "I'd be an idiot not to make this fight...

"That would be the easiest fight to promote in the history of my career."

After previously insisting he was making a joke about the potential fight, Bieber also admitted he didn't fancy his chances if he did grapple with Cruise.

He said: "I'm pretty sure Tom would ... I'm pretty sure he'd whoop my ass in a fight. I'd have to get in good shape, I'm really skinny right now. I think he'd probably be out of my weight class. Because he's big, you know he's got that dad strength."

In his original tweet, Bieber gave no reason for wanting to fight Cruise, but said he wanted to take him on in "the octagon", which is the type of ring used in UFC.

He wrote: "I wanna challenge Tom Cruise to fight in the octagon. Tom if you dont take this fight your scared and you will never live it down. Who is willing to put on the fight? @danawhite ? (sic)"

Bieber has since challenged Cruise star to the latest social media craze, the bottle top challenge, which sees people trying to twist a loose lid off the top of a bottle using a roundhouse kick.