Britain's IBF welterweight champion Kell Brook will challenge undisputed world middleweight champion Gennady Golovkin in London on 10 September.Kazakhstan's Golovkin is unbeaten in 35 professional fights with 32 knockouts.
British middleweight champion Chris Eubank Jr had called out Golovkin, but Brook has got in before him.
"When this fight was offered I accepted in a minute. I'm a huge welterweight and I will carry my speed, accuracy and power through the weights," said Brook.
"This is the ultimate fight, one of the biggest international fights this country has seen in many years.
"You have seen some huge names run from Golovkin but I'm running to him. I'm a special fighter and I will show the world on 10 September."
- Listen: Eubank Sr wanted too much control - HearnSheffield's Brook, 30, is following in the footsteps of compatriot Amir Khan, who also jumped two weights to challenge Mexico's then WBC middleweight champion Saul Alvarez in May.
That fight was at a catchweight of 155lb, five pounds below the middleweight limit, but it is yet to be confirmed if Golovkin will make any concessions.
"I'm very excited to be fighting in front of the great British boxing fans and promise another 'big drama' show," said 34-year-old Golovkin, the WBC, WBA and IBF middleweight champion.
Golovkin has never boxed in Britain before but has beaten two British fighters in world title defences - Matt Macklin was knocked out in three rounds in 2013 and Martin Murray lasted 11 rounds in 2015.
Both Brook and Golovkin have been hunting huge-money fights in recent years but have been repeatedly frustrated.
Head to head Gennady 'GGG' Golovkin Name Kell 'The Special One' Brook Karaganda, Kazakhstan, 8 April 1982 Born Sheffield, UK, 3 May 1986 6 May, 2006 Turned pro 17 September, 2004 WBA, WBC & IBF middleweight Current titles IBF welterweight 35 fights, 35 wins (32 KOs) Pro record 36 fights, 36 wins (25 KOs)
Golovkin won his first world title in 2010 but has struggled to find willing opponents, with Alvarez preferring to give up his WBC belt in May rather than face him.
"It's a tough fight. Kell Brook is fighting one of the most dangerous fighters in the world," Brook's promoter Eddie Hearn told BBC Radio 5 live.
"It will be the toughest test of Gennady Golovkin's career and I think he [Brook] has got a great chance in this fight.
"It's a mega, mega fight for Britain and the world. He's in it because he believes he can win, we believe he can win and he's got as good a chance as anyone.
"Chris Eubank Junior can fight the winner, but you have to strike when fights like this come up.
"It's a real life-changing opportunity to fight Gennady Golovkin and the opportunity to bring him to Britain was one I wasn't going to lose.
"Kell Brook stepped up to the fight and we've got our super fight."
Why did Eubank Jr fight not happen?After defending his British belt against Tom Doran last month, Eubank Jr had said he was keen on a match with Golovkin.
It remains to be seen why that fight fell through.
However, it was because of Eubank Sr's demands: "The problem was that Chris Eubank Sr wanted operational control of the show.
"He wanted to select his own ticket prices, his own undercard… talking about security and hotels. I don't need that hassle, that's what we do, that's what we've done for 30 years."
Golovkin recently suggested Eubank Jr was presented with a contract and failed to sign it, but Eubank Jr denied that.
Eubank Jr said on Twitter on Friday evening: "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."
AnalysisMike Costello, boxing correspondent
"I think I'm among many who've been absolutely blown away by this announcement. This has come from nowhere.
"I was at lunch with Eddie Hearn three weeks ago today ahead of Anthony Joshua's latest world title defence, and he was saying that Chris Eubank Jr and Sr were talking really bullishly about taking on Golovkin and that they really wanted the fight, and Eddie said it was getting closer and closer.
"Why it hasn't happened I can't say, because I'm not involved in the negotiations.
"But this is a mind-blowing announcement because Kell Brook is moving up two weight divisions.
"This is beyond a daunting task for Brook but it's so, so exciting for the sport of boxing, and an indication that Britain is now the centre of the boxing universe."