Priti Patel has approved the extradition of the WikiLeaks co-founder Julian Assange to the US, a decision the organisation immediately said it would appeal against in the high court.

The case passed to the home secretary last month after the supreme court ruled there were no legal questions over assurances given by US authorities over how Assange was likely to be treated.

While Patel has given a green light, WikiLeaks immediately released a statement to say it would appeal against the decision.“Today is not the end of fight,” it said. “It is only the beginning of a new legal battle. We will appeal through the legal system; the next appeal will be before the high court.”

The statement said anyone who cared about freedom of expression should be “deeply ashamed” that the home secretary approved Assange’s extradition.

“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 said.

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

Any appeal is likely to focus on grounds such as the right to freedom of expression and whether the extradition request is politically motivated. Patel had been considering whether the US extradition request met remaining legal tests, including a promise not to execute him.

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

The saga was triggered in 2010 when WikiLeaks published a series of leaks by Chelsea Manning, a former US army soldier, as well as a dump of more than 250,000 US diplomatic cables, some of which were published in the Guardian and elsewhere, containing classified diplomatic analysis from world leaders. The US government launched a criminal investigation into the leaks.

Also in 2010, an arrest warrant for Assange was issued for two separate sexual assault allegations in Sweden. The UK ruled he should be extradited to Sweden.

This prompted the WikiLeaks founder to enter the Ecuadorian embassy in London in August 2012, claiming political asylum. He feared if he was extradited to Sweden, he would in turn be extradited to the US.

He finally left the embassy in 2019. He was arrested in the UK for skipping bail and ultimately jailed. Then extradition proceedings to the US were started against him.

Patel’s decision was met with immediate criticism from campaigners, journalists and MPs.

Caroline Lucas, the Green party MP for Brighton Pavilion, said: “Absolutely shameful that Priti Patel has approved Julian Assange’s extradition to US – this sets a dangerous precedent for press freedom and democracy.

“US authorities are determined to silence him because they don’t like what he revealed.”