UFC boss Dana White said Conor McGregor is "absolutely 100 per cent positive" that he will beat Floyd Mayweather Jr in their hotly-anticipated showdown.

Months of negotiating and trolling finally came to an end on Wednesday as a blockbuster bout for August 26 in Las Vegas was confirmed.

Mayweather, 40, will come out of retirement and put his flawless 49-0 boxing record on the line against UFC champion McGregor in the ring.

McGregor has his doubters as he steps away from what he knows best – the octagon and mixed-martial arts – to go toe-to-toe with Mayweather, but the Irishman is full of confidence, according to White.

"When you talk to this guy it's fascinating. He is absolutely 100 percent positive that he wins this fight," White told ESPN. "I stopped doubting Conor McGregor a long time ago.

"When two guys step into the ring – whether it's the octagon or whatever it is – anything is possible.

"Mayweather is 40 years old, he has always had problems with southpaws. McGregor is 27 years old and a southpaw. Whenever Conor hits people, they fall.

"It's interesting. I'm not saying McGregor is going to win. Not saying Mayweather is going to win.

"The reason McGregor is as big as he is, is because he will fight anybody, anywhere, anytime. He will go after Mayweather and try to knock him out."

White added: "Conor has done a lot of good things for the sport and this company. He wanted this and obviously the fans wanted it too.

"Not only for him, my job is to make fights that the fans want to see."