Tommy Fury has been denied entry to the US ahead of his scheduled news conference with Jake Paul on Wednesday.
The British boxer will face YouTube star Paul on August 6 in New York.
But Fury revealed on social media that US authorities had detained him at Heathrow airport on Tuesday.
“I have no idea why I wasn’t allowed to travel today and neither was my team or my attorney,” he said.
“I’m trying to figure it out. I’m in the middle of training, guys. It’s a big shock for me and my team,” the Manchester-born Fury added.
The fight’s organizers, Most Valuable Promotions, founded by Paul, said in a statement that the news conference had been “postponed” and that they were “surprised” to learn of Fury’s problems.
They confirmed that Fury was allowed to enter the US last month and that Team Fury had given them “plenty of assurances” that Tommy was allowed to travel to the US.
The 23-year-old is the half-brother of WBC heavyweight champion Tyson Fury and is undefeated in his professional career after eight fights.
The boxers share a father, John Fury, who has been unable to enter the US due to a previous criminal conviction.
“As soon as I entered the airport, I was pulled aside and told by a Homeland Security officer who was there that my ESTA [travel authorization] had been denied and that I couldn’t travel to the US for a reason I apparently know.” Fury added.
“I’ve been training for a fight this whole time and that’s all I’ve been doing.”
Paul responded to the news on Twitter, hinting that the fight could still go on.
He said, “Tommy, no matter how hard you try to get out of this fight, I will do everything in my power not to let you get away.
“My team and partners have made it clear what steps you need to take to resolve his latest excuse.”