Anthony Joshua has triggered his clause for a rematch against Oleksandr Usyk, which will take place in the spring of 2022, says Eddie Hearn.
British heavyweight Joshua lost his IBF, WBA and WBO belts to Usyk via unanimous decision at the Tottenham Hotspur Stadium last month.
Usyk has already spoken of his wish to return to his native Ukraine for a rematch with Joshua.
"Joshua is training now, and today we officially triggered the rematch for the Oleksandr Usyk fight, which we will see early next spring," Hearn told DAZN on Saturday.
"So back in the game and looking for him to become a three-time heavyweight champion."
Joshua had appeared set for a unification bout with domestic rival Tyson Fury earlier this year, before an arbitration hearing ruled he had to fight Deontay Wilder again.
Their trilogy fight takes place in Las Vegas on Sunday morning.
Speaking after losing his titles to Usyk, Joshua said: "I've watched the fight, analysed my preparations and identified my mistakes. I've learned my lesson.
"Don't worry about me. My spirit is strong!"
Joshua, 31, previously avenged a surprise defeat to Andy Ruiz Jr - the British fighter's only other defeat of his career - with a unanimous points win in Saudi Arabia in December 2019.