She pleaded guilty to aiding and abetting manslaughter at Leeds Crown Court after filming the attack on Gavin Waterhouse last September.

Mr Waterhouse, 29, from Keighley, later died from a ruptured spleen.

The Crown Prosecution Service said the decision to prosecute the girl was a legal landmark. Mark Masters, 19, and a male, 17, have admitted manslaughter.

All three defendants will be sentenced at a later date.

The message is this: If you stand by and watch your friends committing brutal crimes and video their acts... prosecution may follow

Judith Naylor, CPS

According to the Crown Prosecution Service (CPS), the girl was handed a mobile phone by one of the attackers and was told to "video this".

She then approached Mr Waterhouse and asked him for some money before recording the attack upon him.

Prosecution witnesses said they saw and heard the attackers boasting about what they had just done, saying it was not the first time that they had attacked the victim.

'Cowardly attack'

Judith Naylor, from the CPS, said: "As far as I am aware, this is the first time a suspect in England and Wales has been successfully prosecuted for aiding and abetting murder or manslaughter, for the filming of an inaptly called 'happy slapping' incident.

"The message is this: If you stand by and watch your friends committing brutal crimes and video their acts for yours or for others' amusement your actions will not be ignored by the law enforcement agencies and prosecution may follow."

Neil Atkinson, from the North of England Victims' Association (Neva), said: "It's revolting to think of people celebrating the pain and suffering and eventual death of someone in the way that has obviously taken place in this case."

He said Neva believed the three teenagers should receive long custodial sentences but was concerned about the current "atrocious weak sentencing" of criminals in the UK.

"If they served one to two years in prison, is this sort of person likely to be cured or deterred by that short prison sentence?

"Common sense says that they would not."

Det Supt Paul Kennedy, from West Yorkshire Police, said: "We are pleased with the outcome of this police investigation following what was an unprovoked and cowardly attack. Our thoughts are with Gavin's family."