Saul ‘Canelo’ Alvarez will face Gennady Golovkin in a trilogy fight in September rather than meet Dmitry Bivol in a rematch.
Mexican Alvarez, 31, will return to super-middleweight, where he is undisputed champion, after a shock defeat by Bivol at light-heavyweight.
Alvarez had the option of a rematch with Russian Bivol, but will honour his agreement with Kazakhstan’s Golovkin.
“This is the biggest fight in boxing,” said promoter Eddie Hearn.
The fight will take place on 17 September but no venue has yet been announced.
“These are two men that bitterly dislike each other and want to end this incredible series with a blistering KO,” Hearn added.
“I truly believe this will be the most thrilling fight between these two great champions and it will be fireworks from the first bell to the last man standing.”
Alvarez beat Golovkin in their rematch in 2018 after a controversial draw in 2017, both of which were at middleweight.
Golovkin, who turned 40 in April, has won four successive fights since losing to Alvarez, reclaiming the IBF and WBA (Super) middleweight titles along the way.
Before his defeat by Bivol on 7 May - only the second loss of his career - Alvarez had agreed to face Golovkin on 17 September.
“Unfortunately, we lost, but that doesn’t mean I’m not going to try again. The important thing here is perseverance and we’re going to do it again,” said Alvarez.
“I think those are the two biggest fights in boxing, the fight with Golovkin and the rematch with Bivol.” (BBC Sport)