Floyd Mayweather's August 26 bout with Conor McGregor will be "the biggest fight ever", according to UFC president Dana White.
Former multi-weight world champion Mayweather confirmed a deal has been agreed for a boxing match against mixed martial arts champion McGregor in Las Vegas on August 26.
Mayweather will reportedly be paid over $100m for the contest, which will be broadcast on pay-per-view networks across the world.
And White, in whose organisation McGregor has become a world champion, told Fox Sports: "It's definitely the biggest fight ever in combat sports history, and probably going to be the biggest payday ever.
"Let's just put it this way - all sides involved are pretty happy with their deals!"
The 40-year-old Mayweather announced his retirement after winning his 49th straight bout against Andre Berto, also in Las Vegas in September 2015.
Three months later McGregor won the UFC featherweight title. After splitting two high-profile bouts with Nate Diaz, he then knocked out Eddie Alvarez to add the organisation's lightweight title in December last year.
The fight was first mooted in a series of exchanges on social media, in which McGregor urged Mayweather to step up and sign.
THE FIGHT IS ON. pic.twitter.com/KhW0u3jRft— Conor McGregor (@TheNotoriousMMA) June 14, 2017
Given McGregor's non-existent boxing experience and Mayweather's reputation as one of the finest fighters in history, many observers are already predicting a mis-match but White told ESPN: "(McGregor) is absolutely 100% positive that he wins this fight, and I stopped doubting Conor McGregor a long time ago.
"Floyd Mayweather is 40 years old, he's always had problems with southpaws. Conor McGregor is 27 and he is a southpaw. Whenever Conor hits people, they fall.
"I'm not saying Conor McGregor's going to win. I'm not saying Floyd Mayweather's going to win. But the reason Conor McGregor's as big as he is is he'll fight anybody, anywhere, anytime. He will go after Floyd Mayweather and he will try to knock him out."
White said the fight would be at 154lbs - light middleweight - telling Fox: "It's going to be 12 rounds, they're going to wear 10oz gloves, 154lbs, a boxing ring, Mayweather Promotions will build the undercard - it's going to be a boxing event."
White told reporters in a conference call that there is no language in the contract concerning a rematch.
Mayweather will not be the first boxing champion to take on challenges from opponents steeped in other combat sports.
Muhammad Ali fought a 15-round draw against Japanese professional wrestler Antonio Inoki in Tokyo in 1976, in which each man was allowed to use his own respective talent.