Michelle Stanistreet, NUJ general secretary, said: “A temporary reprieve is clearly preferable to an extradition that would have taken place in the coming days. However, the conditionality around the grounds of appeal, which are contingent on the examination of US government assurances that he will not face the death penalty and has the right to free speech, mean the risks to Assange and press freedom remain stark.

“Assange’s prosecution by the US is for activities that are daily work for investigative journalists – finding sources with evidence of criminality and helping them to get their stories out into the world. If Assange is prosecuted, free expression the world over will be damaged.”

For more than five years Assange has been held in HMP Belmarsh. The US is seeking to prosecute him on 17 charges under the Espionage Act and one on computer hacking, relating to Wikileaks’ publication of the Afghan and Iraq war logs in 2010.

The NUJ has campaigned alongside the IFJ and journalists’ unions around the world in calling for the charges to be dropped, as most relate to activities that are routine activities for many investigative journalists – seeking out witnesses to wrongdoing, encouraging the confidential sharing of evidence, and publishing such information with the pubic.

Ms Stanistreet added: “The nuanced nature of this appeal judgement makes an alternative ending to this situation even more pressing.

“In recent months there has been increasing speculation about some kind of plea deal, to bring this saga to a swift and straightforward conclusion. I urge the US to return to these options.

“Media freedom is under threat all over the world, compassion and common sense from the US Department of Justice would do much to restore Washington’s reputation as a bastion of free expression.”

A further hearing is scheduled for Friday 24 May to determine if the US government’s assurances are “satisfactory”.