Tommy Fury has been denied entry to the US before his scheduled press conference with Jake Paul on Wednesday.
But Fury revealed on social media he had been stopped by US authorities at Heathrow airport on Tuesday.
“I have no clue why they would not allow me to travel today and neither does any of my team or my lawyer,” he said.
“I’m trying to resolve it. I’m in the middle training, guys. It’s a massive shock to me and my team,” Manchester-born Fury added.
Fight organisers Most Valuable Promotions, founded by Paul, said in a statement the press conference had been “postponed” and they were “surprised” to learn of Fury’s issues.
They confirmed Fury was granted entry into the US last month and they had been given “many assurances” by Team Fury that Tommy was allowed to travel to the States.
The 23-year-old is the half-brother of WBC heavyweight champion Tyson Fury and unbeaten in his professional career after eight fights.
The boxers share a father, John Fury, who has been unable to enter the US because of a previous criminal conviction.
“As soon as I entered the airport I got pulled to one side and I was told by a Homeland Security officer that was there, that my ESTA [travel authorisation] had been denied and I wasn’t able to travel to the USA for a reason that I apparently know,” added Fury.
“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 the fight could still go ahead.
He said: “Tommy, no matter how hard you try to get out of this fight, I’m going to do everything in my power to not let you weasel your way out.
“My team and my partners have made it clear the steps you need to take to solve your latest excuse.”