Five teenagers have been arrested over an horrific video showing a young girl apparently being forced to perform a sickening sex act.

The five, all boys aged between 13 and 15, were arrested at 7am today. They were questioned about an allegation of sexual assault and distribution of indecent images of a child at a south London police station.

The video first came to light when a parent brought it to the Croydon Guardian's offices. He was appalled at its content after finding it on his daughter's mobile telephone.

It is thought it was passed around pupils at the school, which we are not naming to protect the identity of the alleged victim, and posted on the video sharing website, You Tube.

After watching the shocking video we immediately called police. We also agreed not to publish any details after detectives said it could jeopardise a trail of evidence.

The video, recorded on someone's mobile phone, showed the young girl, who was dressed in her school uniform, apparently being made to perform a sex act on two teenagers. She is heard asking the laughing boys if she can stop but they are seen carrying on.

One of the boys was also wearing the same uniform but another is dressed in a t-shirt and what appear to be tracksuit bottoms.

The video appears to have been filmed in the school's playground but staff denied this was the case. It is thought the girl's headteacher knew of the video but did not inform police.

A police source said they were taking the case extremely serious and would be asking staff why they did not inform them when they were first made aware of the disturbing film.

A Croydon Council spokesman said: "This was a very serious incident and we are pleased with the progress that the police appear to have made with their investigation.

"Clearly the circumstances are still being investigated by the police and it would not be appropriate for the council to comment further at the present time.

"However, if action on the part of the council is required it will be given proper consideration when all the facts have been determined by the investigation."