Nils Melzer, the UN Special Rapporteur on torture, said Ms Manning, who is being held in a jail in Virginia after refusing to testify about Mr Assange, was being being subjected to an “open-ended, progressively severe measure of coercion fulfilling all the constitutive elements of torture or other cruel, inhuman or degrading treatment or punishment”.

He added: “The practice of coercive detention appears to be incompatible with the international human rights obligations of the United States.”

In regard to Britain’s treatment of Mr Assange, who is being held in London’s Belmarsh prison, where supporters say his health is fading, he said: “Mr Assange’s continued exposure to severe mental and emotional suffering which, in light of the circumstances, clearly amounts to psychological torture or other cruel, inhuman or degrading treatment or punishment.”

The condemnation of the governments’ treatment of the two prisoners was made in separate letters, that Mr Melzer, 49, made public after he said he received no response to concerns he raised with US and British authorities.

Secret war at the heart of Wikileaks Show all 2 1 /2 Secret war at the heart of Wikileaks Secret war at the heart of Wikileaks Julien Assange last night accused those who have attacked him from inside Wikileaks as 'peripheral figures... spreading poison' REUTERS Secret war at the heart of Wikileaks Wikileaks founder Mr Assange storms off the set of the CNN programme

“The continued detention of Ms Manning is not a lawful sanction….and should be discontinued and abolished without delay,” he said on Twitter.

Of the letter released about Mr Assange’s treatment, he said: “My official findings, supported by medical experts, unquestionably provide “reasonable ground to believe” UK officials contributed to Assange’s psychological torture or ill-treatment.”

Though they have never met, the lives of Ms Manning, 32, and 48-year-old Mr Assange, became inextricably linked after the one-time army intelligence analyst provided a wealth of materials about the US-led invasion and military operation in Iraq to Wikileaks, the whistleblower organisation Mr Assange founded in 2006.

Among the most damning material was video footage that showed two US AH-64 Apache helicopters attacking buildings in Baghdad in 2007, and then closing in a group of people. Among the people were children and journalists.

Chelsea Manning: Jailed US analyst walks free after refusing to testify to WikiLeaks grand jury

“Oh, yeah, look at those dead bastards,” one US airman can be heard to say. The attack killed at least a dozen people.

Ms Manning served seven years for leaking the video, much of that time spent in solitary confinement. She was detained again in the spring of 2019 after refusing to testify against Mr Assange at a grand jury that was established in Virginia to support a prosecution of him.

“I will not participate in a secret process that I morally object to, particularly one that has been used to entrap and persecute activists for protected political speech,” she said.

Mr Assange was arrested on April 11 at the Ecuadorean Embassy in London, after US prosecutors in the Eastern District of Virginia unsealed a criminal case against him, alleging he conspired with Ms Manning to commit computer intrusion.

Mr Assange, whose organisation also published information about Guantanamo Bay, the US state department and Democratic Party emails allegedly obtained by Russian hackers, is preparing to fight his case at an extradition hearing, due to begin in February.

The US has charged him with 17 offences, most of the them under the Espionage Act. The US alleges he helped Ms Manning hack a Pentagon computer network, something his supporters have denied is true.

After he was arrested, a lawyer for Mr Assange, Barry Pollack, said: “These unprecedented charges demonstrate the gravity of the threat the criminal prosecution of Julian Assange poses to all journalists in their endeavour to inform the public about actions that have been taken by the US government.“

Mr Assange, whose health is said to be worsening according to friends and supporters who have visited him in jail, faces up to 175 years in jail if convicted.

An arrest warrant from Sweden, relating to allegations of sexual assault, has since been dropped.