Undefeated boxer Floyd Mayweather and mixed martial arts champion Conor McGregor, two of the most popular athletes in their respective sport, have agreed to a August 26 fight in Las Vegas.

The fight is expected to gross millions of dollars with Mayweather a solid favourite. Mayweather, 40, retired in 2015 with an unblemished 49-0 professional boxing record while boastful Irishman McGregor, 28, is the UFC's lightweight champion.

The two fighters have gone back and forth for nearly a year regarding a potential showdown with both men previously stating they each wanted $US100 million for the fight.

Mayweather is widely regarded as one of the best defensive fighters of all time, and prides himself on his ability to evade punishment with his skilful movement about the ring while making adjustments against opponents on the fly.

McGregor, the UFC's reigning lightweight champion and former featherweight champion, is known as much for his bravado as for his explosive knockout power inside an MMA octagon.

The Irishman became the first UFC fighter to hold two belts simultaneously when he knocked out Eddie Alvarez in New York last November for the lightweight championship.

The UFC stripped McGregor of his featherweight belt shortly after that fight when he announced he would be taking some time off after he and long-time partner Dee Devlin had their first baby in May.

Known for his devastating striking and highlight-reel knockouts, McGregor is also quick and irrational on his feet, but few would back him to beat one of the most complete boxers of all time.

His UFC rival, Nate Diaz, who is 1-1 with McGregor in their two fight series, is at the very least giving his former foe a puncher's chance.

“My take on a boxing match between the two...I think McGregor has got a good chance to make something happen in two or three rounds,” Diaz told CSN Sports. “If anything, two or three rounds. But he’s got an amateur style where he’s only got good movement, good punches for six to eight minutes and I think that’s too amateur for Mayweather.”

“I’m just saying he’s got a puncher’s chance -- if anything. But as far as skill and technique, he’s got an amateur style and anybody can be good for nine, six, seven minutes. Anybody. They all have a puncher’s chance. This guy [Mayweather] is ready to go round after round and he’s a hard person to get a hold of. I think it’d be a rough one [for McGregor], but who knows?”