Anthony Joshua has finally got a December date for his clash with Kubrat Pulev.
But Oleksandr Usyk has thrown a spanner in the works ahead of a potential undisputed clash with Tyson Fury next year.
The WBA, IBF and WBO heavyweight clash between Joshua and Pulev is pencilled in December 12 at the O2 in London.
That is a week before WBC champion Fury hopes to face Deontay Wilder and the plan is for both Brits to square off early next year for all the titles.
But now Ukraine star Usyk is demanding his mandatory WBO shot against Joshua before he can face Fury.
“Usyk has been the WBO mandatory challenger for more than one year," Usyk's co-promoter Alexander Krassyuk said.
“He has agreed to let AJ versus Pulev happen, with regards that the winner fights him next. This statement is still solid.”
Usyk is due to face Derek Chisora later this year in his second fight at heavyweight after moving up from cruiserweight where he unified all four belts.
Krassyuk added: “Our plan is A) beat Derek Chisora B) push AJ to comply with WBO rules or vacate the title C) defeat AJ or any other WBO champion or contender and become the heavyweight champion. Simple and practical.”
Joshua would face a decision to either drop the WBO belt or make Fury wait for their highly-anticipated showdown.
Fury looked set to be held to a mandatory defence against Dillian Whyte but the latter's knock out defeat by Alexander Povetkin solved that issue.
The Gypsy King said he would fight Joshua in December but he first must face Wilder in a contracted rematch.
Joshua's promoter Eddie Hearn is hopeful of being able to have fans at the December 12 date at the O2.
He has requested that his September show – likely to be headlined by rising light-heavyweight star Joshua Buatsi – could be a pilot event in the reintroduction of spectators for the government.
Meanwhile, Frank Warren – who promotes Fury – has said heavyweight hope Daniel Dubois would be ready to face Usyk for the vacant WBO belt if Joshua vacated.
Dubois has risen to No. 3 in the WBO rankings and Warren has no fears about stepping him up against former world cruiserweight king Usyk.
He said: “It wouldn't be an easy fight but it would be one I'd back Daniel to win.”