Wikileaks founder Julian Assange is threatening to sue The Guardian for libel after claims in a book published by the newspaper about its collaboration with him.

The prospect of the defamation action comes as the 39-year-old Australian is due appear in court in south-east London this morning to fight extradition to Sweden, where he is wanted for questioning about allegations of sexual assault against two women.

Mr Assange's battle to overturn a European arrest warrant issued by the Swedish authorities will enter a crucial phase at Belmarsh magistrates' court when a judge begins hearing two days of evidence. Mr Assange's lawyers are expected to argue the Swedish prosecutors have incorrectly filed the case and that the proceedings are politically motivated.

The Australian may retain the support of those who see his case as a line in the sand in the fight for greater transparency in government and commerce. But cracks are appearing in his relationships with established media.

Mr Assange is understood to be angry at claims in a book published by The Guardian which chronicles the events leading up to the publication of thousands of secret American diplomatic documents.

The Australian is believed to be upset at a claim that he initially refused to remove the names of informants mentioned in Afghan war documents, allegedly saying they would "deserve it" if they were killed as a result of the leaks.

Last week, the WikiLeaks Twitter account, which is understood to be written by Mr Assange, posted a message which read: "The Guardian book serialisation contains malicious libels. We will be taking action."

Guardian News & Media, the publisher of The Guardian, said it had not received any official notification of action against it by Mr Assange. A spokeswoman added: "The irony of an organisation dedicated to the free and open flow of information threatening to sue a newspaper will be lost on no one."

Mr Assange, who is living at the Norfolk country home of a supporter after being released on bail during the court proceedings, is unlikely to hear before March whether his attempt to fight the extradition request has been successful.

Subsequent appeals to the High Court by either side are likely, meaning Mr Assange will probably remain in Britain for a number of months.

Belfast Telegraph