Ukrainian boxer Oleksandr Usyk celebrates his victory over Britain s Tyson Fury in Riyadh. Photo: AFP
The Ukrainian, 37, became the first heavyweight to hold the IBF, WBO, WBA and WBC belts when he beat Tyson Fury on a split decision in Saudi Arabia last month.
With Usyk now contracted to a rematch with Britain's Fury, he is not in a position to defend the IBF crown against interim champion and mandatory challenger Daniel Dubois.
That means Dubois' expected all-British fight against Anthony Joshua at Wembley in September could now be for the vacant title.
Usyk said in a video posted on social media: "Anthony and Daniel, listen. I know the IBF title is important to you. It is my present to you on September 21."
Britain's Lennox Lewis was the last man to unify the major heavyweight belts -- three at the time -- prior to Usyk, in 1999.
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