Kell Brook will move up two weight divisions to face WBC, WBA and IBF middleweight champion Gennady Golovkin on 10 September at London's O2 Arena.
The unbeaten Sheffield fighter had been expected to put his IBF welterweight title on the line against WBO holder Jessie Vargas in a unification bout later this year.
In an unexpected twist however, Brook will instead take on Kazakh Golovkin, widely regarded as the best fighter at 160lbs and who has earned a reputation as a knockout specialist, having stopped 32 of his 35 opponents.
"This is the fight and the moment I have been waiting for for some time," said Brook.
"We've tried and tried to lure the best to come and fight me and now we've got the best of the best - the number one pound for pound fighter in the world is coming to the UK."
Regardless of the result against Golovkin Brook (36-0, with 25 KOs) will retain his status as IBF welterweight champion, a title he has defended against three underwhelming opponents since beating Shawn Porter by a close decision in California two years ago.
He has been craving a showdown against a big-name opponent although he and his handlers appeared content to continue campaigning at 147lbs.
Brook, though, had no hesitation in boldly agreeing to move up in weight.
He added: "When this fight got offered to me I accepted within minutes and I can't wait.
"Everyone knows I'm a huge welterweight and I know I will carry my speed and accuracy and even more power through the weights.
"This is the ultimate fight, one of the biggest international fights the country has seen in many years - you have seen some huge names run from GGG but I'm running to him. I'm a special fighter and I will show the world on September 10."
Golovkin, above, (35-0, with 32 KOs) has stopped his last 22 opponents, including Brook's compatriots Matthew Macklin and Martin Murray on his rise to the top of the middleweight rankings.
"I'm very excited to be fighting in front of the great British boxing fans and promise another 'Big Drama Show' against Kell Brook," he said.
The 34-year-old, who is based in the United States, had seemingly been earmarked to next fight Chris Eubank Jr, who reacted with dismay to Friday's announcement.
He wrote on Twitter: "I haven't been shown one contract since my fight with GGG was announced. I'm still ready & waiting to sign. Fighting Brook is a joke #2Small."
Eddie Hearn, who promotes both Brook and Eubank Jr, said: "I cannot tell you how excited I am for this mega fight.
"Right here you have the number one welterweight in the world against the number one middleweight in the world - 36-0 vs 35-0, two of the best on the planet, it's what big time boxing is all about."
I haven't been shown one contract since my fight with GGG was announced. I'm still ready & waiting to sign. Fighting Brook is a joke #2Small
— Chris Eubank Jr (@ChrisEubankJr) July 8, 2016
