LONDON (REUTERS) - The World Boxing Organisation (WBO) on Saturday (May 22) ordered Anthony Joshua to defend his heavyweight title against mandatory challenger Oleksandr Usyk after the Briton's bout with domestic rival Tyson Fury was thrown into doubt.

Joshua, who also holds the International Boxing Federation and World Boxing Association titles, was set to face Fury - who holds the World Boxing Council belt - on Aug 14 in Saudi Arabia.

But Fury was ordered by an American arbitrator on Monday to fight Deontay Wilder for a third time before Sept 15.

The WBO said it had sent a letter to Joshua's promoter Eddie Hearn on Wednesday to "show cause" within 48 hours why the WBO should not order a fight with Usyk, asking for confirmation of the Joshua-Fury bout.

Hearn asked the WBO for an extension on Friday, but the request was denied.

Joshua and Ukrainian Usyk have until May 31 to reach an agreement for the fight, the WBO said, or purse bids will be called, opening the process to any promoter who wants to bid for the bout.

If the fight goes to a purse bid, Joshua would receive 80 per cent of the bid, while former undisputed world cruiserweight champion Usyk would get 20 per cent.

"Such calculation is based upon the average purse of each fighter's last three bouts," the WBO said, adding that the minimum bid would be US$1 million (S$1.33 million).