Anthony Joshua's next fight is "very likely" to be against Deontay Wilder or Alexander Povetkin, according to the Briton's promoter Eddie Hearn.
Joshua unified the WBA, IBF and WBO heavyweight titles last weekend by outpointing Joseph Parker at the Principality Stadium and immediately called out WBC titlist Wilder for a bout that would see all four major world belts on the line.
Any collision course may have to be put on the backburner as the WBA ordered Joshua to enter into negotiations with mandatory challenger Povetkin.
Hearn told Sky Sports: "The WBA called the mandatory yesterday for AJ to face Povetkin next.
"We have a 30-day window to negotiate - at this stage it means it's very likely Anthony's next fight will be between Wilder or Povetkin."
Sky Bet make Joshua the odds-on favourite at 4/9 to get the win if he fights Wilder, who is the 15/8 outsider.
With the mandatory challenge set, Joshua will either need to arrange a fight, vacate the belt or agree a deal that would see Povetkin temporarily step aside and agree a showdown at a later date.
Povetkin, whose only defeat in a 35-fight professional career came against Wladimir Klitschko in October 2013, produced a chilling knockout of David Price on the undercard of Joshua-Parker in Cardiff.
The Russian's promoters World Boxing said on Twitter: "Povetkin is now a mandatory contender of @anthonyfjoshua
"He was appointed by WBA. Now there are 150 days to arrange the fight, it must take place.
"Before the fight with Sasha (Povetkin), Joshua has no right to fight with anybody else. We have 30 days to negotiate@EddieHearn"
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