By Alan Jones and Sian Harrison, PA Media

London, UK – Pleas for Julian Assange to be freed from prison are being stepped up amid warnings that the WikiLeaks founder is suffering "severe psychological abuse."

The United States administration is appealing against a UK court decision in January not to extradite him, but no date has been set for the hearing. Assange remains in the UK's Belmarsh prison.

His legal team said on Wednesday that permission to appeal has been granted by the High Court on a limited basis, allowing only "narrow, technical grounds."

Assange’s fiancée, Stella Moris, said: "Six months ago, Judge Vanessa Baraitser blocked the extradition of my partner, Julian Assange, because consigning him to the US prison system would have amounted to signing his death warrant. That should have been the end of it.

"The case is rotten to the core, and nothing that the US government can say about his future treatment is worth the paper it is written on. This is a country whose agents plotted to kill Julian on British soil, who harried his solicitors and stole legal documents; who even targeted our six-month-old baby.

"I am appealing directly to the Biden government to do the right thing, even at this late stage. This case should not be dragged out for a moment longer. End this prosecution, protect free speech and let Julian come home to his family.

"The current administration admits that the Trump Department of Justice lacked independence. It seems inconceivable that President Biden would want to continue with this case – because Julian’s freedom is coupled to all our freedoms, and no democratic society can ever make journalism a crime.

"If the Biden Administration does not end this now, the case will limp through the courts while Julian remains in prison indefinitely, unconvicted, suffering and isolated, while our young children are denied their father."

Speaking to journalists outside the Royal Courts of Justice on Wednesday, having visited Assange in Belmarsh earlier in the day, Moris said: "The High Court delivered its decision that it will allow limited permission for the US Government to appeal January’s decision to block Julian’s extradition.

"That means he is still at risk of extradition where he faces a 175-year prison sentence and, according to (Judge Baraitser), is certain to lose his life if he is extradited.

"The US Government should have accepted the Magistrates’ Court’s decision – instead, it keeps this case going.

"The case is itself falling apart. The lead witness of the US Department of Justice now admits that he lied in exchange for immunity from US prosecutors. The lawyers of Julian were spied on, their offices were broken in to. Even our six-month-old baby was targeted while he was in the embassy.

"And now the High Court has limited the grounds on which they are to appeal, so the case is falling apart.”

She added: "(US Attorney General) Merrick Garland has egg on his face because of the decision to use a witness who perjured himself in order to try to imprison Julian and keep him in prison.

"I’m appealing to the Biden administration to do the right thing. This appeal was taken two days before the Trump administration left office, and if the Biden administration is serious about respecting the rule of law, the First Amendment and defending global press freedom, the only thing it can do is drop this case."

Moris said Assange is "very unwell," adding: "He is happy when he sees us … it’s not representative when I see him because obviously it’s the one respite he gets.

"Belmarsh prison is a terrible place. Just yesterday, another prisoner was found dead in his cell. The suicide rate is three times higher than the average UK prison and it is a daily struggle.

"He won his case in January. Why is he even in prison? Why is he even being prosecuted?

"There is no legal case against him, all there is is an indictment based on lies."

A group of doctors representing over 250 medical professionals from 35 countries wrote to US leader Joe Biden on Wednesday, urging him to drop charges against Assange under the Espionage Act.

They said: "Mr. Assange continues to suffer serious, life-threatening effects of the psychological torture he has been subjected to for more than a decade."