Billy Joe Saunders' son punches and kicks his opponent Willie Monroe Jr in the groin at weigh-in

The weigh-in for Saunders and Willie Monroe Jr’s WBO middleweight title fight in London was interrupted on Friday, when little Billy decided to attack big Willie

Little Billy attacked big Willie at the Friday weigh-in (Getty)

The weigh-in for Billy Joe Saunders and Willie Monroe Jr’s WBO middleweight title fight in London was interrupted on Friday, as the English boxer’s son struck his father’s opponent in the nether regions.

After the two men had weighed in for the fight, Saunders’ son, Billy Joe Jr, stood on the scales between the two men while they were posing for photographs.

Monroe reached out to playfully tousle the youngster’s hair, only for the boy to react by punching him in the groin.

Saunders fights Monroe Jr this weekend (Getty)

A security guard and a member of Monroe’s entourage both reacted quickly, attempting to pull the boy away, only for him to aim a kick at the American middleweight before they could be separated.

Saunders ignored the entire incident, although later took to Twitter to explain the action of his son.

“My son has been taught when a stranger puts their hands on them they don’t know, punch and run away self-defence,” he explained.

Saunders will defend WBO middleweight title against Monroe Jnr on Saturday having been told a unification fight with Gennady Golovkin is essentially a "done deal".

The 28-year-old fights his American opponent at London's Copper Box Arena just hours before Golovkin defends his WBA, WBC and IBF titles in his Las Vegas superfight with Saul Alvarez, and should he succeed, expects to fight the winner.

Saunders' promoter Frank Warren has long been in discussions with the handlers of the 35-year-old Golovkin and had hoped to secure a date with him in Kazakhstan earlier this summer.

Saunders wants to fight the winner of Golovkin vs Canelo (Getty)

Each also consider the Kazakh the favourite to overcome Mexico's Alvarez, and believe that should he do so, he will next agree to a match-up for all four world middleweight titles in London in December.

Golovkin fought in London last year when defending his titles against Saunders' stablemate Kell Brook, and Warren said: "If Golovkin wins, it's a done deal. It's very close. If he delivers, I'll deliver.

"It's just a matter of coming through it. Willie Monroe Jnr is going to be difficult for him: he's not here to make up the numbers. Both of these guys can see a pot of gold there.

"It'll probably be December: that's what we're looking at. If he wins, whatever happens, he's fighting in December, and we're hoping that'll be against the winner. If Alvarez wins, that might not be until next year, depending on when he's going to fight, but certainly as far as Golovkin's concerned, that'll be done very quickly.

"Bill wants to test himself, be in there with the best. If he wins the fight, gets the winner of those two: it's a fantastic position to be in, he can write his own cheques. I can't think of any Briton who's gone and beaten someone of that calibre."

There has been a bad-tempered buildup to the fight (Getty)

Warren and Saunders believed they were close to a fight with Golovkin in the latter's home country but after a disappointing performance in his defeat of Daniel Jacobs, he chose to rest and then the Alvarez fight became a reality.

The 30-year-old Monroe Jnr instead provides a significant test for Saunders, who has been trained for the first time by Dominic Ingle, but aside from an Alvarez victory, he and Warren do not believe he will miss out twice.

"We had a contract for it," said the promoter, 65.

Warren is confident the fight will take place (Getty)

"The reason it didn't happen was after the fight with Jacobs, they decided that he needed a break. And if I was working with him, I'd have given him a break. He's probably one of the most active fighters in the world at championship level.

"(An Alvarez match-up) depends when he's going to fight. He fights on Mexican holidays: that's where he earns all his money. He's not a guy who fights three or four times a year; he's been fighting twice a year.

"Whoever wins, (Saunders has) got an important piece of the jigsaw providing he beats Willie Monroe Jnr. If he comes through the fight, he's got the other belt. And who's there for the winner to fight? It's got to be him. That's where we're at."

(Additional reporting by Press Association.)