Peter Fury, head trainer of WBO middleweight champion Savannah Marshall, has warned that his boxer is easily the most dangerous puncher in women's boxing.

Marshall will collide with IBF, WBA, WBC champion Claressa Shields in an undisputed showdown on September 10 at the O2 Arena in London.

The British fighter has knocked out 10 of her 12 opponents.

"She's got devastating power in both hands. By far the hardest female puncher in history, going back and forward," Fury said to Sky Sports.

"I've got somebody who's six foot, who can make various weights with ease, which is a gift in itself, who's as bodily strong as some men, with weights, cardio. She's a gifted athlete. The best of Savannah will be too much for anybody."

Fury is training Marshall harder than she's ever trained in her life. 

"I said to Savannah in this camp, it's going to be harder now than it's ever been. I said it's going to be brutal to be honest, and I'm going to treat you brutal as well," Fury said.

"So you need to understand when it's like this, there's no room for messing around, get it done, [if] it's wrong, do it right. That's what it is. That's just the way this camp is. Once you know that, you know what you're dealing with. When you fight to unify belts, it's a serious matter. If there's 100 percent, I don't want 99.8, I want 100. All I tell my fighters and all I say to Savannah, 'you're in for hell and high water'. 'This is the hardest fight of your career, train like it'. So she's prepared for absolute Armageddon."

Marshall remains the only fighter to defeat Shields in a boxing ring - by securing a decision when the two boxers fought as amateurs.

Shields would move forward to capture gold in two Olympic games. As a pro, she's captured world titles in three weight classes.