The list of top-ranked fighters who have said, “Thanks but no thanks” to middleweight champion Gennady Golovkin is far more impressive than the actual 35-0 record with 32 KOs of the WBC, WBA, IBF and IBO champion. But in a sudden and strange turn of events that is sure to generate controversy, undefeated IBF welterweight champion Kell Brook agreed on Friday to jump from the 147-pound division to 160 to face “GGG.”

Golovkin recently was frustrated when Canelo Alvarez chose to give up the WBC middleweight title rather than meet him in a unification bout. Promoter Tom Loeffler, who represents Golovkin, then entered negotiations with top contender Chris Eubank Jr., who turned down the deal presented by his English promoter Eddie Hearn.

“It was on Wednesday, and it looked like that wasn’t going to happen,” Loeffler told Newsday. “Hearn called Kell Brook, and he was interested in taking the fight. It took us two days, under the same terms he was offering Eubank, to close the deal with Kell Brook.”

Brook (36-0, 25 KOs) is taking a huge leap in class but will not give up his welterweight title, and Golovkin, who has scored 22 straight stoppages, will be fighting on Brook’s home turf Sept. 10 at O2 Arena in London. The bout will be on pay-per-view SkyBoxOffice in the United Kingdom but will air on regular HBO Championship Boxing in the United States.

“It made sense,” Loeffler said. “You have an undefeated welterweight champion fighting an undefeated middleweight champion. You have the speed of the welterweight coming up matched with the power of Gennady. I think it will be a very entertaining fight and a huge event.”

Brook, who never ventured above 147, said in a statement: “When this fight got offered to me, I accepted within minutes. I’m a huge welterweight, and I know I will carry my speed and accuracy and even more power through the weights.”

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Many decried the bout as a mismatch. Brook stands 5-9 compared to the 5-10 ½ Golovkin. But such jumps are not uncommon in boxing. Sugar Ray Leonard came off a three-year layoff to fight for the first time at middleweight against Marvelous Marvin Hagler.

“Knowledgeable fans will recognize this is the biggest fight we could have made for Gennady,” Loeffler said. “Whenever you can fight another undefeated champion, it’s a big statement.”

Loeffler said Golden Boy Promotions, which represents Alvarez, felt the “timing wasn’t right and wanted to build up” a fight with Golovkin.

“I think it will happen next year, and when it does happen, it’s a huge event,” Loeffler said of Alvarez. “But certainly, the fight with Brook is something nobody on our side can underestimate. Any plans I make for Gennady in the future go out the window if Gennady doesn’t continue to win.”