Assange, who is currently serving time at a high-security prison in Britain, was to answer questions from a judge at Spain's National Court in Madrid, testifying by videoconference from Westminster Magistrates Court in London, his legal team said.

Spain's top criminal court is investigating whether Undercover Global Ltd, which was responsible for security at the embassy, spied on Assange and passed on information to the United States.

The case is key to Assange's efforts to fight an extradition request by the US Justice Department which is pushing to have him put on trial for leaking hundreds of thousands of secret US military and diplomatic documents in 2010.

"The case being investigated in Spain states that Mr Assange has been subjected to widespread interference on a massive scale by the American authorities, violating his confidential communications with his lawyers, among other rights," his legal team said.