WikiLeaks called it a “dark day” for press freedom and British democracy.

The Australian is being held at Belmarsh prison in London after mounting a lengthy battle to avoid being extradited.

Ms Patel made the decision on Friday, although Mr Assange’s legal team will mount an appeal.

Home Secretary Priti Patel has signed an order to extradite Julian Assange to the US to face espionage charges, the Home Office has confirmed.

A Home Office spokesperson said: “Under the Extradition Act 2003, the Secretary of State must sign an extradition order if there are no grounds to prohibit the order being made. Extradition requests are only sent to the home secretary once a judge decides it can proceed after considering various aspects of the case.

“On June 17, following consideration by both the Magistrates’ Court and High Court, the extradition of Mr Julian Assange to the US was ordered. Mr Assange retains the normal 14-day right to appeal.

“In this case, the UK courts have not found that it would be oppressive, unjust or an abuse of process to extradite Mr Assange. Nor have they found that extradition would be incompatible with his human rights, including his right to a fair trial and to freedom of expression, and that whilst in the US he will be treated appropriately, including in relation to his health.”

A WikiLeaks spokesman said: “This is a dark day for press freedom and for British democracy. Anyone in this country who cares about freedom of expression should be deeply ashamed that the home secretary has approved the extradition of Julian Assange to the United States, the country that plotted his assassination.

“Julian did nothing wrong. He has committed no crime and is not a criminal. He is a journalist and a publisher, and he is being punished for doing his job.

“It was in Priti Patel’s power to do the right thing. Instead she will forever be remembered as an accomplice of the United States in its agenda to turn investigative journalism into a criminal enterprise.

“Foreign laws now determine the limits of press freedom in this country and the journalism that won the industry’s most prestigious prizes has been deemed an extraditable offence and worthy of a life sentence.

Julian Assange wins ruling in his battle against extradition to US

“The path to Julian’s freedom is long and tortuous. Today is not the end of the fight. It is only the beginning of a new legal battle.””

Agnes Callamard, Amnesty International secretary general, said: “Allowing Julian Assange to be extradited to the US would put him at great risk and sends a chilling message to journalists the world over.

“If the extradition proceeds, Amnesty International is extremely concerned that Assange faces a high risk of prolonged solitary confinement, which would violate the prohibition on torture or other ill treatment."

Former minister David Davis said he did not believe Assange would have a fair trial in the US and called for extradition rules to be rewritten.

The Conservative MP and former Brexit secretary said on Twitter: “This extradition treaty needs to be rewritten to give British and American citizens identical rights, unlike now.”

Labour MP Richard Burgon said it was a “disgraceful decision”, warning the WikiLeaks founder faced the rest of his life in prison for his journalistic work.

Julian Assange: Home Office to extradite Julian Assange to the US to face espionage charges