WikiLeaks founder Julian Assange was given permission to have a full appeal over his extradition to the United States after arguing at London's High Court on Monday he might not be able to rely on his right to free speech at a trial.

Two judges at the High Court said they had given him leave to have a full appeal to hear his argument that he might be discriminated against on the basis the Australian-born Assange is a foreign national.

Hundreds of protesters had gathered outside the court ahead of what was a key ruling after 13 years of legal battles, with two judges asked to declare whether they were satisfied by US assurances that Assange, 52, could rely on the First Amendment right if he is tried for spying in the US.

The news was met outside the court by an eruption of cheering and singing. Assange's legal team had said if he lost he could be on a plane across the Atlantic within 24 hours.

His lawyer Edward Fitzgerald had told the judges they should not accept the assurance given by US prosecutors that Australian-born Assange could seek to rely upon the rights and protections given under the First Amendment, as a US court would not be bound by this.

"We say this is a blatantly inadequate assurance," he told the court.

Fitzgerald had accepted a separate assurance that Assange would not face the death penalty, saying the US had provided an "unambiguous promise not to charge any capital offence".

The US said its First Amendment assurance was sufficient. James Lewis, representing the US authorities, said it made clear that Assange would not be discriminated against because of his nationalityin any US trial or hearing.

Asaange's legal team were buoyant after the decision was made. Fitzgerald said it could be months before the appeal was heard.