Google, Facebook and Twitter have been ordered by the police to remove photographs purporting to show one of James Bulger's killers.

The police intervention came after the attorney general threatened to prosecute those who uploaded pictures claiming to be of Jon Venables, now 30, to the internet.

Police served the three web giants with the injunction that bans the purported identification of Venables and Robert Thompson, who were released with new identities in 2001 after being jailed for the murder of Bulger in Liverpool 20 years ago.

Legal experts said the breach could result in a landmark mass contempt prosecution by the government, following a number of recent cases that brought cyberspace into direct confrontation with the law.

A spokeswoman for the attorney general's office said police had requested that Twitter, Facebook and Google "assist with the removal of material in breach of the terms of the order" and that the process was ongoing.

The photographs are believed to have begun circulating online on 14 February and some were still available on Monday.

The attorney general, Dominic Grieve, issued a sharply worded statement on Monday warning that he would launch contempt proceedings against "a number of individuals" who published the online pictures.

Twitter broke its silence about the issue in a Commons home affairs select committee hearing on Tuesday afternoon.

Keith Vaz, the Labour MP and chair of the committee, asked why Twitter was not taking down the photographs that are in breach of the injunction.

Sinéad McSweeney, Twitter's director of public policy in Europe, the Middle East and Africa, said she did not wish to be drawn into commenting on individual accounts.

She added: "We work with law enforcement here in the UK. We have established points of contact with law enforcement in the UK where they communicate with us about content, they bring content to our attention that is illegal, and appropriate steps are taken by the company. You may read into those words what you wish in context of the current [issue]."

McSweeney, who appeared alongside officials from Google and Facebook, said Twitter could not be expected to proactively monitor what is published on its social network across the globe each day. She added: "It's important that people increasingly understand that online is no different to offline: what is illegal offline is illegal online."

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