While there is nothing concrete, or even remotely tangible, in place, Floyd Mayweather Jr. continues to fan the flames of a possible bout with MMA fighter Conor McGregor.

Mayweather, who said earlier this month he was the one who started rumours of a possible fight between the two champions, shared a mock promotional poster on Twitter late on Sunday night.

McGregor posted a similar image on Twitter three weeks ago.

Making the fight come to fruition presents plenty of difficulties, with a 151-pound boxer potentially squaring off against a 170-pound MMA fighter. 

Mayweather, who could get over $100 million for the fight, 'retired' for the third time in his career after defeating Andre Berto last September. The former welterweight champion has never been beaten in 49 fights. But McGregor, who is the UFC featherweight champion with a record of 19-3, would present a different challenge.

Mayweather, 39, is a patient tactician as a boxer, a complete contrast from McGregor's aggressive, yet careful, style. 

As both sides continue to tease the bout, UFC president Dana White has called it an impossible prospect.