Anthony Joshua will get the chance to win back his IBF, WBA and WBO heavyweight titles in a rematch with Andy Ruiz Jr later this year.
Joshua's promoter Eddie Hearn announced the contracted rematch clause was triggered on Tuesday and the second fight will take place in November or December, with a venue still to be confirmed.
The Briton suffered a shock first defeat of his career when he was stopped by Ruiz Jr in the seventh round at Madison Square Garden in New York at the weekend.
Hearn tweeted on Tuesday evening: "After meetings with AJ, Rob Mc (Joshua's trainer Rob McCracken) and the management team in NY, we have today triggered the contracted rematch clause with Andy Ruiz Jnr. The fight will take place in Nov/Dec at a venue to be confirmed shortly."
Joshua was on the receiving end of one of the biggest shocks in the history of heavyweight boxing on his United States debut.
He was knocked down twice in the third round and twice more in the seventh, forcing referee Mike Griffin to stop the contest.
Joshua had targeted a rematch straight after the defeat, calling the loss a "minor setback".
"I didn't even ask about a rematch clause because I was so confident I was going to win," he said. "If it's there, we'll go again and I'll get the titles back. I'm going to beat him up."
Hearn revealed that the UK was the likeliest venue for the rematch.
"It's now down to the rematch and winning that fight," he said after the fight. "He must win that fight and he will do anything he can to regain his title. To get back to the heights he has been, he must win that rematch."