Conor McGregor

Australia's Horn, who this month beat Manny Pacquaio to win the WBO welterweight title in Brisbane, has called out the 40-year-old five-division world champion.

But Mayweather ruled out any prospect of a fight with the former Queensland schoolteacher in front of a packed press conference in London on Friday, saying he'll hang up his gloves after facing McGregor on August 26 in Las Vegas.

"This is my last fight," Mayweather said.

"I've just done years and years and years of just fighting and travelling.

The pair have been trading verbal blows for months and a deal has been a long time coming despite both men stressing they were keen for the fight to take place.

However, that all changed on Wednesday after the Nevada State Athletic Commission approved a request put in by Mayweather Promotions to host a boxing match in late August.

Soon after, both Mayweather and McGregor took to social media to confirm the fight will take place, a move that will send shockwaves through both boxing and MMA.

Speaking exclusively to Sporting News cageside at Madison Square Garden ahead of a historic night of fights, Gallen said the tall poppy syndrome that pervades Australian sport would quickly put any McGregor-like character back in his place.

Case in point: Jarryd Hayne.

“Being at the top of the NRL, it’s sometimes a little bit disappointing because I saw the way Jarryd Hayne spoke when he came back and it was all very much about himself – I didn’t mind it,” said Gallen.

“I thought it was confidence, but other people take it the wrong way.