Pupils set up lunch-break Fight Club and post the shocking videos on YouTube

In another scene, a teenager doubles over in agony after apparently taking a blow to the stomach. His attacker is ready to follow up with a left hook.

And all the time, other pupils are filming everything that happens on their mobile phones before posting the footage on YouTube.

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These shots are the disturbing evidence of a fad which has spread through a boys' school where pupils have set up their own Fight Club, based on the ultra-violent film of the same name starring Brad Pitt. In the film, disaffected young men fight each other in illegal bare-knuckle bouts.

In this case, the arena is the playground of Hitchin Boys' School in Hertfordshire, where painted lines for tennis and football are clearly visible.

The fights, held in school hours, have also taken place in darkened rooms.

On the video-sharing website YouTube, a disclaimer appears at the beginning of one battle, stating: "All fighters in this video volunteered to fight and are willing to do so."

But there is concern that children could be seriously injured or worse if the practice is not stamped out.

Keith Wadsworth, head of the comprehensive "specialist technology college", said: "It looks really horrible and we will be looking into it.

"The trouble is, once it's on the web, you cannot control it and it's in the public domain."

In a worrying development, pupils from John Henry Newman School, a mixed Roman Catholic comprehensive in nearby Stevenage, have put footage of similar clashes on the internet, suggesting the craze is spreading.

One shows a pupil crouching behind the lid of a school bin while another boy runs over and performs a drop-kick on him.

Mick Brookes, of the National Association of Head Teachers, said: "Boys fighting at school is as old as the hills, but not in this systematic way of setting up a club.

"It's interesting to hear it has spread from one school to another, and it needs to be snuffed out quickly.

"Perhaps it could be termed as affray, in which case the police could be involved."

A Hertfordshire Police spokesman said the force had not yet been contacted by the schools.

"We have schools officers, so they could look into it and if they believe a crime has been committed, we would investigate."

The pupils have now removed the footage from the internet.

A YouTube spokesman said: "Sadly, there is a tiny minority who try to break the rules.

"On YouTube, these rules prohibit content like pornography and gratuitous violence.

"When people see content they think is inappropriate, they can flag it and our staff then review it.

"If the content breaks our terms, then we remove it, and if a user repeatedly breaks the rules, we disable their account."