World heavyweight champion Deontay Wilder claims he has agreed to fight Britain's Anthony Joshua in the UK.
The American, 32, holds the WBC title and wants a unification bout with Joshua, the WBA, IBF and WBO champion.
He tweeted: "The $50m offer for him to fight me next in the US is still available. Today I even agreed to their offer to fight Joshua next in the UK.
"If he prefers the fight in the UK, the ball is in their court. It's up to them to choose."
Wilder's manager Shelly Finkel later told ESPN they had officially accepted terms to a fight in the UK.
Joshua and promoter Eddie Hearn have yet to publicly respond, though Hearn has previously criticised the Wilder team for appearing to do their business via social media, insisting "deals are made in the boardroom, not on Instagram".
Finkel said: "Deontay sent an email to Joshua [Sunday] night, and I sent one [Monday] to Barry Hearn and Eddie telling them that we officially accept the offer to fight under the terms they gave us and to send us the contract."
Should Joshua, 28, fight the American next, the winner would become the first heavyweight to hold all four recognised world titles at once.
Russian Alexander Povetkin is the WBA's mandatory challenger and is close to a deal to fight Joshua, although Joshua could face Wilder first if Povetkin is able to fight the winner.