Tyson Fury has been ordered to fight Deontay Wilder for a third time, casting doubt over a proposed August super bout with Anthony Joshua.
Fury, 32, won the WBC world heavyweight title from Wilder in a rematch in February 2020 after an initial draw.
The pair were due to fight again but delays caused by the Covid-19 pandemic prompted Fury to seek an alternative.
On Monday a US judge ruled in mediation that Wilder has a right to face Fury for a third time before 15 September.
The American's team had long argued their fighter had a contractual right to a third fight with Fury.
But after delays caused by the pandemic led to a July 2020 contest being postponed, Wilder's team sought to move the bout to 2021.
Fury refused the 2021 date and said he would move on. That prompted mediation overseen by American judge Daniel Weinstein, who has now sided with Wilder and set a 15 September deadline for a third contest.
"Until we can reach an accommodation with Deontay Wilder's people, I can't say that [Fury's fight with Joshua] will definitely happen," Fury's promoter Frank Warren told BBC Radio 4's Today Programme.
"But we are working hard to get that over the line. We were talking last night and hopefully we can do that. If not then Tyson, if he wishes, will go to the fight with Wilder. They're the only two options."
Fury's US promoters Top Rank did not wish to comment on the development when contacted by BBC Sport.
But Top Rank founder Bob Arum told ESPN that Allegiant Stadium in Las Vegas has been reserved for a Fury-Wilder fight on 24 July.
"We're not paying Wilder to step aside," said Arum. "It's better to get rid of him and go about our business. We can make the Fury-Joshua fight for November or December."
Just 24 hours before Monday's arbitration hearing, Fury confirmed he had agreed to fight fellow Briton Joshua - holder of the IBF, WBA and WBO belts - for all four heavyweight world titles in Saudi Arabia on 14 August.
Joshua's promoter Eddie Hearn has also said several times that the August bout was in place, with terms agreed.
The mediation outcome does not necessarily mean a Fury-Joshua fight cannot take place in August but fresh deals will now need to be struck for it to do so.
That could mean Wilder being offered money to allow Fury-Joshua to take place.
"It'll cost quite a bit of money. He will want as much as possible so I think it'll be about the negotiation if we go down that road," said Warren.
Saudi authorities are reportedly putting forward around £106m just to stage the Fury-Joshua contest.
Pay-per-view sales and ticket sales will serve to substantially add to the total revenue of the contest.
If it goes ahead, Joshua versus Fury will be the first bout in heavyweight history where all four world titles are on the line.
'Expect a deal' - analysis
BBC Sport's Luke Reddy
Just 24 hours after Fury said he had been in conversation with Saudi organisers and agreed to seal the deal, this Wilder news will come as a blow.
How big of a blow is not yet clear.
With so much money to be made from Joshua v Fury, one would think a deal will be struck with Wilder whereby he steps aside and perhaps even gets told he can face the winner in 2022.
It may suit him to do so given he can pocket a sizeable sum and prepare against an opponent of his choosing short-term - he has been out of the ring for 15 months.
There were already countless moving parts to a Joshua-Fury deal, with three promoters, several broadcasters, four sanctioning bodies and a host venue all needing to co-operate to get the fight done.
But any postponement would be highly damaging for the sport given so many fans from outside boxing's hardcore support will tune in to watch it. It is a golden opportunity to draw new people into the sport, and fights with such a crossover appeal are rare.
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