A schoolgirl faces a custodial sentence for filming a brutal and fatal attack on a mobile phone.

The conviction was described as a legal landmark yesterday because it is thought to be the first successful prosecution of someone who filmed a “happy-slapping” attack but did not also take part physically.

The 15-year-old, who cannot be named for legal reasons, pleaded guilty to aiding and abetting the manslaughter of Gavin Waterhouse last September when she filmed the attack on a mobile phone.

The Crown Prosecution Service described the prosecution as bold. It is intended to send a message to youths involved in the growing trend for happy slapping.

The girl admitted filming an attack on Mr Waterhouse, 29, of Keighley, West Yorkshire, in which he was punched and kicked. He died of a ruptured spleen. She has already been sent to a secure unit and will be sentenced at a later date. In theory she faces the same maximum sentence as for the offence that she aided and abetted – which is life in this case – although that is unlikely.

Judith Naylor, the Crown advocate with West Yorkshire Crown Prosecution Service and junior counsel in the case, said: “This is believed to be 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.”

She said that the CPS had made a landmark decision in advising that the girl be charged.

Ms Naylor said: “We have seen and heard of many instances in recent months and years where youths have been encouraged by their peers to attack innocent people without words ever having been used. The message is this: if you stand by and watch your friends committing brutal crimes and video-record their acts for yours or others’ amusement your actions will not be ignored by the law enforcement agencies and prosecution may follow.”

Grahame Stowe, a criminal lawyer with Grahame Stowe Bateson, a law firm in Yorkshire, said: “More than ever before young people are turning to extreme violence, in some cases for no reason other than entertainment. This case illustrates once again the gross callousness some teenagers are capable of.” He added that it was a bold move to prosecute someone “who, in a literal sense, does not have blood on his or her hands”. He said: “The principle that videoing a criminal offence taking place can be considered aided and abetting could have some serious ramifications.”

The girl was accused alongside Mark Masters, 19, and a 17-year-old youth, who cannot be named for legal reasons, who pleaded guilty at an earlier hearing at Leeds Crown Court to the manslaughter of Mr Waterhouse. On Wednesday she pleaded guilty at the same court to aiding and abetting, counselling and procuring the unlawful killing.

Judge James Stewart, QC, remanded the girl to Newton Aycliffe secure unit while reports are prepared on her. She will be sentenced with Masters and the other youth at a later date.

Mr Waterhouse was found dead at his flat after being injured in the violent assault. A postmortem examination showed that his death was due to internal injuries and that he had suffered a ruptured spleen.

Tom Bayliss, QC, for the prosecution, said that the teenager had filmed about 15 seconds of violence – an attack that included punches and kicks to the victim. The three ran off and one witness described the two males laughing about what happened. The girl did not know Masters and the other youth before the incident but continued to associate with them afterwards.

The footage of the violence was later shown to others even after it was known that Mr Waterhouse had died, although the earlier hearing was told that the recording no longer existed.

Detective Superintendent Paul Kennedy, of West Yorkshire Police, said: “We are pleased with the outcome of this police investigation following what was an unprovoked and cowardly attack.”

Times Recommends

• The fourth plinth art project: famous for 60 minutes
• Rubik has a new puzzle to irritate the nation
• Is it Radio 4 or Radio Bore?