In the clip, you see a boy, dressed in a school uniform, appearing to be ganged up on by bullies.

He backs off, refusing to fight, but one of the group, a girl, seems to land a savage slap to his face.

Things begin to heat up and others join in on the action.

One boy, dressed in white, makes an advance, before exclaiming, what sounds like, “I’ll smash you.”

But he doesn’t even get halfway through his sentence before the the bloke, who seems to be on the receiving end of the attention, unleashes a brutal left hook that appears to shake the jaw of his opponent.

The white shirted teen buckles over and drops to the floor, perplexed by the blow.

His friends gasp and scream: “Oh my god, Callum, Callum.”

The footage, believed to be filmed in the Merseyside area, was posted to Twitter by Darren Turley on Thursday.

The clip has been retweeted over 4,000 times and liked by over 10,000 viewers.

Darren wrote: “Nothing gives me more pleasure than seeing bullies get this treatment."

Social media users reacted to the video with sympathy for the so-called picked on boy who was forced into retribution, and praised the quality of his punch.

Sean Berry said: “Yes. What a hit son. What a hit.”

Another user said: “Never condone violence, but when it's self preservation and defence you are protected. Perfect timing.”

Rob Hughes said: “I don't like violence but I'd be lying if I said this wasn't a bit pleasing.

“With a bit of luck neither Calum or his lynch mob will feel quite so emboldened tomorrow.”

Paul Jones said: “I've been that kid. Backing away, being harassed and fearful. The release in standing up for yourself is massive.”