Floyd Mayweather has revealed he is "officially out of retirement" in order to fight UFC champion Conor McGregor.
A bout between the five-weight boxing world champion Mayweather and UFC's biggest star has been rumoured for weeks.
Mayweather, 40, has previously stated he would only return to the ring to fight the Irishman and, on Sunday (AEDT), he gave the clearest indication yet that it could happen.
Speaking in Liverpool on the latest leg of his UK tour, Mayweather said: "I'm officially out of retirement for Conor McGregor, today.
"If he's really real, like he's says he is, if you really want to fight, just sign the paper.
"Conor McGregor, stop blowing smoke up everyone's arse, if you want to fight, let's make the fight happen."
McGregor has obtained a boxing licence granted by the California State Athletic Commission, which would allow him to box in the US state, as part of his pursuit of a cross-sport showdown with Mayweather.
However, the 28-year-old McGregor is still contracted to the UFC.
Mayweather retired in September 2015 with a 49-0 record while McGregor became UFC's first simultaneous two-weight champion in November when he beat Eddie Alvarez for the lightweight belt.
Mayweather had earlier played down talk of an agreement between the two camps, posting on Twitter: "There seems to be several rumours floating around media recently however, let the record show, there hasn't been any deals made in regards to a fight between myself and any other fighters.
"I am happily retired and enjoying life at this time. If any changes are to come, be sure that I will be the first to let the world know."