Kell Brook will challenge Gennady Golovkin for his WBA Super, WBC and IBF world middleweight titles on 10 September. In a stunning development, IBF welterweight champion Brook, 29, will step up two weights to challenge the undefeated Kazakh, who has knocked out 32 of his 35 opponents to date.

Brook, also undefeated, has successfully defended his welterweight title three times after his victory over Shawn Porter in August 2014 and looked primed to defend his belt against Jessie Vargas in September.

Instead, the Sheffield-born fighter makes the remarkable step up to challenge knock-out artist Golovkin, who himself looked nailed on to take on another opponent in Chris Eubank Jnr.

"This is the fight and the moment I have been waiting for for some time," said Brook, who will retain his status as the IBF's 147lb-champ," Brook said, Sky Sports report.

"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 No 1 fighter in the world is coming to the UK.

"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.

Gennady Golovkin
Golovkin undefeated in his professional career. Getty

"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."

Brook stepping up to challenge the fearsome middleweight champion draws parallels with Amir Khan's decision to step up to 155lbs against Saul 'Canelo' Alvarez, a decision that resulted in the Briton being brutally knocked out.

Brook, who has never fought above 147lbs, was recently involved in a social media spat with the man who looked certain to be his next opponent in Vargas. While the two traded barbs over the contract negotiations between the two camps, indications where an official announcement was just around the corner.

Similarly, Golovkin appeared ready to defend his titles against Eubank Jnr, insisting he would be happy to come to the UK to take on the brash challenger.

Instead, the 34-year-old will arrive on these shores to meet Brook in what will now be one of the most eagerly-anticipated fights of the year.