‘Crimes against humanity’: UN body calls for release of Guantánamo inmate

The UN working group on arbitrary detention (UNWGAD), also declared the UK, among other countries, was "jointly responsible for the torture and cruel, inhuman or degrading treatment of Mr Zubaydah" over his more than 20 years in detention.

The UN working group is a quasi-judicial body, issuing legal judgments on behalf of the international community, but they are not binding, nor does it have the power to enforce its findings. It is made up of five legal experts from around the world: the current panel consists of lawyers, law professors and former judges drawn from Malaysia, New Zealand, Ukraine, Ecuador and Zambia.

Friday marks the first time an international body has referred to the 21-year-old prison camp as a potential crime against humanity, the first time such a body has ruled against the US for Abu Zubaydah's detention, and the first international case finding against the UK, Morocco, Thailand and Afghanistan, all of whom are deemed complicit in arbitrary detention, rendition and torture.

The decision also found Pakistan, Poland and Lithuania to be part responsible: Pakistan for participation in his arrest and rendition, and Poland and Lithuania for hosting black sites. The European Court of Human Rights has previously ruled against Poland and Lithuania for their participation in the web of secret detention facilities and rendition flights.

"The UK has been found legally responsible for 'complicity' in our client's torture and ongoing unlawful detention, and reparations should follow," Duffy said. "This can include offers of relocation, recognition and apology, rehabilitation and compensation."

She added: "We need to reckon with the fact that the 'war on terror', as waged for 20 years, has failed. But we cannot pretend to learn lessons from it while perpetuating its most notorious wrongs."

Zubaydah is one of three "forever prisoners" who have not been charged and not been recommended for transfer.