Julian Assange's lawyer in Britain has accused Swedish authorities of secretly planning to extradite the WikiLeaks founder to the United States.

Attorney Mark Stephens told German newspaper Die Zeit that he believed Swedish officials were cooperating with US authorities with an eye to extraditing Assange as soon as the Americans have built a criminal case against him.

"We are hearing that the Swedish are prepared to drop the rape charges against Julian as soon as the Americans demand his extradition," he said, citing sources in Washington and Stockholm.

Mr Stephens called the Swedish charges against his client a "holding case" to buy time until the United States can prosecute him themselves over WikiLeaks' mass release of classified US documents.

He said Assange did not believe he would receive a fair trial in Sweden which was why he was fighting his extradition from Britain.

The Australian has been living at a supporter's country estate in England since being released on bail on December 16 after his arrest by British police on a Swedish warrant.

Mr Stephens said he believed the "last station" of an extradition to Sweden would be "a high-security prison in the United States".

Assange's lawyers released documents on Tuesday saying that if the Australian is extradited to Sweden there is a "real risk" he will face extradition or illegal rendition to the United States, where he could be detained at Guantanamo Bay or elsewhere and subject to the death penalty.

A British judge ruled on Tuesday that Sweden's bid to have him extradited would be heard in full on February 7-8.

Swedish authorities want to question Assange about charges brought by two women that he sexually assaulted them, but the 39-year-old says the extradition attempt is politically motivated and linked to WikiLeaks' activities.

The whistleblowing website has released classified documents about the wars in Iraq and Afghanistan and from US diplomats stationed around the world.

A US court has reportedly subpoenaed the Twitter accounts of four WikiLeaks supporters as part of a widening criminal investigation into the leaks.

- AFP