As previously reported on BoxingScene.com, Kell Brook will move up two weight divisions to face WBC, WBA, IBO and IBF middleweight champion Gennady Golovkin on September 10 at London's O2 Arena.
The unbeaten Sheffield fighter had been expected to put his IBF welterweight title on the line against WBO titlist Jessie Vargas in a unification bout later this year.
However, in an unexpected twist, Brook will instead take on 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.
Brook (36-0, with 25 KO wins) retains 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.
Brook follows in the footsteps of fierce rival Amir Khan, who himself jumped up from welterweight to middleweight earlier this year to take on then WBC champion Saul 'Canelo' Alvarez. That fight, which Khan lost by a brutal sixth-round knockout, was fought at a catchweight of 155lbs and it is unknown whether Golovkin will make a similar concession in weight.
Golovkin (35-0, with 32 KO wins) 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.
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."