Anthony Joshua has been ordered to defend his WBA heavyweight title against mandatory challenger, Luis Ortiz.
The governing body released a statement telling both parties they have 30 days to come to an agreement, or the fight will go to purse bids.
The decision came just over 12 hours after Wladimir Klitschko announced his retirement, ruling out a rematch with Joshua, who added the WBA to his IBF title with the stunning 11th-round stoppage at Wembley Stadium in April.
Pulev welcomes AJ in Wales
The IBF mandatory challenger Kubrat Pulev is happy to challenge Anthony Joshua in Cardiff
The IBF's mandatory challenger Kubrat Pulev is already in line to challenge 'AJ', but the WBA's January resolution confirmed that Ortiz would face the Wembley winner.
A statement on wbaboxing.com read: "According to the resolution sent on January 11th, 2017, to both parties, it was reported that the winner of the Joshua-Klitschko fight, held on April 29th, 2017, had to face Ortiz after 120 days.
"95 days have passed and, for this reason, the Championships Committee ordered to negotiate a fight between the British and Cuban boxers.
"Both parties were duly notified that, as of August 3rd, they have 30 days to reach an agreement or the fight will be called to purse bids."
Joshua and promoter Eddie Hearn have been in Las Vegas, with the proposed Klitschko rematch having previously been pencilled in for the T-Mobile Arena on November 11.
Ortiz's manager, Jay Jimenez, insists the unbeaten Cuban should fill the void.
"We are next," he told boxingscene.com.
"We have no problem fighting on that date of November 11th in Las Vegas. Our mandatory takes precedence over Pulev's and they know that."