MADRID: US prosecutors are secretly preparing a case against WikiLeaks founder Julian Assange for publishing a cache of sensitive diplomatic cables, his lawyer Baltazar Garzon said Wednesday.

The 41-year-old Australian has been holed up in Ecuador's embassy in London since June after losing a legal battle to avoid extradition to Sweden to face questioning over rape and sexual assault claims.

He fears that if sent to Sweden, he will be re-extradited to the United States to stand trial for spying for publishing the leaked US diplomatic cables that embarrassed governments around the world.

Spain's Garzon, known for once pursuing Chile's former dictator Augusto Pinochet, said he was convinced that a "secret grand jury" in the United States had launched an investigation into Assange and WikiLeaks.

He said he based this belief on statements made by people who have testified in the probe of US Army Private Bradley Manning for passing the trove of classified documents to WikiLeaks.

"The procedure (against Assange) exists. We are going to ask US authorities to tell us if they have launched a procedure against WikiLeaks that affects Julian Assange," Garzon told a meeting with foreign journalists in Madrid.

"I can already tell you that they will not respond. It is true that there are no formal charges (in the United States) against Julian Assange but from my experience charges can be laid in just 24 hours."

Garzon said Assange is not against going to Sweden to be questioned in the rape and sexual assualt case but wants guarantees that he will not be extradicted to the United States.

In 2010 WikiLeaks, a whistle-blowing website, released hundreds of thousands of classified diplomatic documents on its site, including about the Afghan and Iraq wars.

Garzon was banned from the judiciary in Spain in February after standing trial accused of exceeding his authority in investigating a political corruption case. The judge has said he is working for WikiLeaks for free.