WikiLeaks founder Julian Assange's appeal against a UK court ruling that approved his extradition to the United States to face trial for breaking national security laws will begin on July 9, a judicial spokesman said Tuesday.

Assange, 52, won his bid to challenge the ruling last month, and the two-day hearing will now take place at the High Court in London next month.

The Australian publisher is wanted by Washington for publishing hundreds of thousands of secret US documents from 2010 as head of the whistle-blowing website WikiLeaks.

Had he lost at last month's hearing, Assange -- who has become a figurehead for free speech campaigners -- could have been swiftly extradited after a five-year legal battle.

In written submissions for the hearing, lawyer Edward Fitzgerald, representing Assange, accepted as "unambiguous" US government assurances that he would not face the death penalty.

But he queried whether his client could rely on the First Amendment of the US Constitution, which covers freedom of speech and freedom of the press, at trial.

James Lewis, representing the US government, told the court Assange's conduct was "simply unprotected" by the First Amendment.

It does not apply to anyone "in relation to publication of illegally obtained national defence information giving the names of innocent sources to their grave and imminent risk of harm", he submitted.

Assange has been detained in the high-security Belmarsh prison in London since April 2019.

He was arrested after spending seven years holed up in Ecuador's London embassy to avoid extradition to Sweden, where he faced accusations of sexual assault that were eventually dropped.

US authorities want to put Assange on trial for divulging US military secrets about the wars in Iraq and Afghanistan.

He is accused of publishing some 700,000 confidential documents relating to US military and diplomatic activities, starting in 2010.

The United States has accused Assange under the 1917 Espionage Act, which his supporters warn mean he could be sentenced to 175 years in prison.