Anthony Joshua is a big baby, says rival Kevin Johnson

Anthony Joshua and Kevin Johnson fight in London on Saturday

British heavyweight prospect Anthony Joshua has been called "a big baby" by American Kevin Johnson before their meeting in London on Saturday.

Olympic champion Joshua has 12 knockout wins from 12 professional fights but Johnson is a step up in class.

The 35-year-old has six defeats and one draw from 36 fights but has not been stopped since turning pro in 2003.

"He's nothing but a grown man in a diaper - I will expose that to the world," Johnson told BBC Radio 5 live.

"I don't rate him at all. He has no business being on my level. His corner will either throw in the towel or he'll be stopped before they can even get their hand in the air."

Asked how Joshua had managed to dispose of 12 opponents in only 22 rounds if he was just "a big baby", Johnson replied: "He ain't fought nobody yet.

"He did the same thing that my grandmother could do if you give her the same opponents."

Joshua and Johnson box on the 'Rule Britannia' card at the O2 Arena, which is headlined by Kell Brook's IBF welterweight defence against Frankie Gavin.

Opponents Johnson has taken the distance include Britons Dereck Chisora and Tyson Fury and former heavyweight world champion Vitali Klitschko.

In contrast, Joshua has not been past the third round in any of his professional bouts.

Having dispatched Raphael Zumbano Love only three weeks ago, the 25-year-old is prepared for a tougher test against Johnson, who has been in the UK for two months.

"Kevin Johnson is very experienced and very defensive," said Joshua, who won super-heavyweight gold at London 2012.

"I want do a job on him for sure but I am not going to get carried away. I am going to break him down bit by bit."