Justin Bieber v Tom Cruise is the celebrity fight we didn’t know we were desperate to see until earlier this week.

Bieber called Cruise out via Twitter - seemingly for no reason whatsoever - saying he wanted to take on the Hollywood star in a MMA fight.

"I wanna challenge Tom Cruise to fight in the octagon,” the pop star tweeted. "Tom if you don't take this fight your [sic] scared and you will never live it down. Who is willing to put on the fight? @danawhite?"

Before UFC president Dana White could respond, Conor McGregor piped up and attempted to goad Cruise into accepting the bout.

“If Tom Cruise is man enough to accept this challenge, McGregor Sports and Entertainment will host the bout,” the Notorious tweeted. “Does Cruise have the sprouts to fight, like he does in the movies? Stay tuned to find out!”

Bieber, 25, is half the age of Mission Impossible star Cruise, who turns 57 next month.

He also has a height and estimated reach advantage over the actor.

However, Cruise is an all-action hero in the movies and keeps himself in excellent shape for a man in his late 50s.

Perhaps Bieber has just watched Mission: Impossible - Fallout, Cruise’s latest blockbuster, and realised the man he wants to fight is still a physical specimen.

Why? Well, because he’s taken back his offer to fight Cruise, claiming he was only joking.

"Man... I was just playing,” Bieber told TMZ reporters, “it was just a random tweet I'm pretty sure Tom would probably whoop my ass in a fight.

"I'd have to get super in shape, I'm really skinny right now.

"I think he'd probably be out of my weight class. He's got that dad strength."

What a let down!

Sounds like Bieber has lost his bottle, despite giving it the big one on Twitter earlier this week.

It’s too late to say you’re sorry this time, Justin.