LONDON (Reuters) - Lawyers for Julian Assange said on Thursday they will argue that the WikiLeaks founder cannot be sent from Britain to the United States to face spying charges because a treaty between the two countries bans extradition for political offences.

Assange, 48, faces 18 counts in the U.S. including conspiring to hack government computers and violating an espionage law. He could spend decades in prison if convicted.

At London’s Westminster Magistrates’ Court, his lawyer Edward Fitzgerald outlined some of the evidence Assange’s team will put forward at the full extradition hearing due to start on Feb. 24, saying they could call up to 21 witnesses to testify.

“We say that there is in the treaty a ban on being extradited for a political offence and these offences as framed and in substance are political offences,” he told the court.

Other arguments would feature medical evidence, public denunciations by leading U.S. political figures, and details from the case of Chelsea Manning, an ex-intelligence analyst who was convicted by a U.S. Army court-martial in 2013 of espionage and other offences for leaking secret cables to WikiLeaks.

There would also be information from an investigation led by a Spanish judge into “revelations about bugging of conversations with his lawyers” during Assange’s long stay in the Ecuadorean embassy in London.

Assange, who appeared by videolink from prison on Thursday, is due to be interviewed by a Spanish judge when he appears in person at the same London court for a private hearing tomorrow over the allegations.

He spent almost seven years holed up in cramped rooms at the embassy where he fled in 2012 to avoid extradition to Sweden where he was then wanted for questioning over allegations of rape which were later dropped.

He was dragged from the embassy in April and jailed for 50 weeks for skipping bail before the U.S. launched its extradition request.

Due to the mass of evidence, the full hearing to decide the extradition issue is now set to last for up to four weeks, rather than one as originally planned, Judge Vanessa Baraister said, with Fitzgerald adding it involved “difficult, important and profound” matters.

Baraister also seemed unimpressed when Clair Dobbin, representing the U.S. authorities, asked for the case to be delayed until April, saying the lawyer earmarked for the case would not be available for the extended hearing.

“My impression was the (U.S.) government was anxious for this case to remain on track and not to be derailed,” said Baraister, who said the next hearing would take place on Jan. 23 and the timetable would go ahead as planned.