Anthony Joshua's next fight will be against Russian Alexander Povetkin, promoter Eddie Hearn has said.
Joshua was on Tuesday morning given a 24-hour deadline to sign to fight Povetkin or face being stripped of a portion of his world heavyweight title.
Hearn has been negotiating for a unification bout with WBC titlist Deontay Wilder, but the WBA has lost patience and wants him to face its own mandatory challenger.
WBA president Gilberto Mendoza said in a statement: "Today the WBA requested an answer from Anthony Joshua's team regarding his fight with the mandatory challenger Alexander Povetkin.
"The WBA have allowed over a month extension to negotiations with Povetkin and also ongoing discussions with Deontay Wilder.
"It appears the Wilder team have not returned the contract for the fight and therefore we are requesting a date for the Joshua versus Povetkin fight with immediate effect."
The demand did not appear likely to cause Joshua's team too many issues given negotiations with the Wilder camp appear to be deadlocked and former champion Povetkin represents the next-best bet for a stadium-filling autumn bout.
Hearn had indicated he was close to agreeing a deal with Wilder, but the American's team claim to be holding out for "clarifications".
An irked Hearn told Sky Sports News on Tuesday afternoon: "We can't do a lot more than we're doing. I think Anthony's frustrated that he feels people feel that he might not want the fight (with Wilder). He wants that fight more than anything.
"We sent the contract nearly nine days ago now. We're not even necessarily expecting a signed contract back - we just want your comments.
"If I wanted a fight, my fighter wanted a fight, and we received a contract, we would be back with the comments within 24 hours. Nothing makes sense here at all.
"(Team Wilder) emailed me on Sunday and said, 'We will be back with our comments on Friday'. Why do you need another five or six days?"
He continued: "They know we're under pressure from the WBA and it all feels like a big game.
"This Wilder fight is one that has to get done and has to get signed. It's a fight we desperately want.
"But at the same time we're not going to give up a belt and we're not going to lose a belt to the WBA for ordering the mandatory, and if Povetkin's ready to go, then that will be next."
Povetkin, who held the WBA's 'regular' title between 2011 and 2013, beat Briton David Price in his last fight in March on the undercard of Joshua's win over Joseph Parker in Cardiff.
The Principality Stadium would be favourite to host the contest between Joshua and Povetkin, confirmation of which would delay a clash with Wilder until at least next year.