Anthony Joshua has been ordered to fight Oleksandr Usyk after hopes for a summer showdown with Tyson Fury faded this week.
The WBO on Saturday sent a letter ordering the unified heavyweight titleholder to fight Usyk (18-0), the sanctioning body's mandatory challenger.
While Joshua (24-1) holds the WBO, IBF and WBA belts, Fury (30-0-1) claimed the WBC title from the previously unbeaten Deontay Wilder (41-1-1) in their February 2020 rematch following a draw in their initial bout.
On Monday, a judge in the United States ruled that the dethroned champion had the right to face the Briton for a third time before September 15.
Two days later, the WBO sent Joshua's promoter Eddie Hearn a letter giving him 48 hours to show cause why it should not mandate a title defense against Usyk.
On Friday, Hearn asked the body for an extension until Monday, but the WBO denied that request Saturday.
The WBO gave the Joshua and Usyk camps 10 days to finalise an agreement for a fight, or the body will order a purse bid.
Should that happen, the letter said, Joshua would receive 80 per cent of the minimum $1million bid and Usyk 20 per cent.