WikiLeaks founder Julian Assange’s extradition trial could blow out to a month as more witnesses come forward and evidence mounts to more than 40,000 pages.

Assange appeared via videolink at a case management hearing in London’s Westminster Magistrates Court on Thursday.

Unshaven and wearing glasses, the Australian sniffled during his video appearance from Belmarsh prison, where he’s been held since his April 11 arrest at the Ecuadorian embassy in London.

He was living there in asylum for almost seven years.

Assange has been charged in the US with 17 counts of spying and one count of computer hacking in relation to WikiLeaks’ release of thousands of classified Pentagon files regarding the Iraq and Afghanistan wars.

The files were allegedly provided by former US army intelligence analyst Chelsea Manning.

Some of them documented American war crimes, and the 48-year-old’s legal team will argue his actions were political, not criminal, and that a treaty between the US and UK forbids extraditions for political offences.

“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,” barrister for Assange Edward Fitzgerald QC told the court.

Mr Fitzgerald added that due to mounting evidence, up to four weeks would be needed for his client’s trial – instead of the five days originally scheduled.

The barrister described bundles dossiers of evidence relating to the espionage charges against Assange, along with documents relating to Manning.

There’s also evidence from varied sources including Assange’s psychiatrist, legal proceedings in Spain over alleged spying on Assange from inside the Ecuadorian embassy, and evidence relating to members of the US administration who publicly denounced the Australian.

Up to 21 witnesses are to be involved in the trial, which is set to start in late February.

Mr Fitzgerald also raised the prospect of further evidence coming to light in coming months.

But defence lawyer Clair Dobbin said an extension from five days to four weeks would mean lead prosecutor James Lewis QC, who was not present on Thursday, may not be available for the full trial.

“We hadn’t understood that it was going to be suggested that four weeks be set aside,” she said.

District Judge Vanessa Baraitser rejected Dobbin’s suggestion of putting off the trial until April, saying a further delay could “impact the process”.

“My impression was the (US) government was anxious for this case to remain on track and not to be derailed,” the judge said.

She ordered the prosecution and defence to sort out a trial date before the next case management hearing on January 23.

-AAP