The WBA ordered Joshua to “immediately” sign for a fight with mandatory challenger Povetkin this morning after the Brit failed to agree terms with Wilder for a unification bout.
The authorities had given Joshua a one month extension but they have now grown tired at a lack of progress.
Hearn has previously stated that talks were ongoing with Povetkin and they were discussing potential options in case the Wilder clash fell through.
But they will now be forced to implement the contingency plan and confirm the fight by the end of the week.
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“The WBA have mandated the Povetkin fight,” Hearn told Sky Sports News.
“Losing a belt is not an option.
“If it’s Povetkin it will be September for AJ, then we fight Wilder after if he signs the contract.
“AJ wants the fight but there’s not much more we can do at present.”
Povetkin knocked out David Price in his last outing
WBA president Gilberto Mendoza said: “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.”
Povetkin earned a shot at the belt after beating David Price on the undercard of Joshua against Joseph Parker.