The government has approved the extradition of Julian Assange to the US, where he faces espionage charges.
The Home Office confirmed Home Secretary Priti Patel had signed the extradition order for the WikiLeaks founder.
Assange is wanted in the US over an alleged conspiracy to obtain and disclose national defense information after WikiLeaks published hundreds of thousands of leaked documents relating to the Afghanistan and Iraq wars.
He has always denied any wrongdoing.
Assange has 14-days to appeal the decision and his legal team said they will be appealing.
“Today is not the end of the fight. It is only the beginning of a new legal battle. We will appeal through the legal system,” WikiLeaks said in a statement.
“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.”
Assange has been held in Belmarsh high-security prison since 2019 when he was dragged out of the Ecuadorian embassy in London, where he had taken refuge.
Originally a judge ruled that Assange should not be deported, saying his mental health problems would make him a suicide risk if convicted.
However, this decision was overturned on appeal.
In a statement, a Home Office spokesperson said: “On 17 June, following consideration by both the Magistrates Court and High Court, the extradition of Mr Julian Assange to the US was ordered.
“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.”
Amnesty International said the decision to extradite Assange “sends a chilling message to mobile”.
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In March, Assange married his partner, Stella Morisafter they were given permission to wed last year.
She was joined by the couple’s sons Gabriel, four, and Max, two, and Assange’s father and brother, Richard and Gabriel Shipton.
Mrs Assange said: “The home secretary has approved sending Julian to the country that planned to murder him. Julian has exposed US government criminality.
“The home secretary is condoning not only the criminality committed by the US government against Julian, but also those US government crimes exposed by WikiLeaks.
“Julian is a political prisoner. We will use every avenue to appeal this decision. I will dedicate every waking hour to fight for justice until he is free.”