The US court granted the petition for a writ of habeas corpus for Asadullah Haroon Gul, who has been detained without charge by the US since 2007. The decision marks the first time in a decade a Guantanamo detainee's imprisonment has been ruled unlawful.

"We are thrilled for Asad. A federal court has finally affirmed what Asad has known for so long: he should be home with his family, and his detention is unlawful," Mark Maher, Gul's lawyer at Reprieve US, said in a statement.

"This is a landmark ruling. For 20 years, successive US administrations have asserted their right to imprison people indefinitely, without charge or trial. Guantanamo was built on the shakiest of legal foundations, and that has never been more clear than it is today."

Earlier this year, US government lawyers had argued that Washington could continue to detain the Afghan citizen even as the country was pulling out of Afghanistan, asserting that he had a past affiliation with members of al-Qaeda.

He was accused of being a member of Hezb-e-Islami (HIA), a militia that fought against the allied invasion of Afghanistan. HIA entered into a peace agreement with the then-US backed Afghan government five years ago.

Wednesday's court decision comes a week after Guantanamo's Periodic Review Board cleared Gul for release, although he remains at the prison and the US has not provided a country to which he could be released.

Gul's brother, Roman Khan, said the court's ruling was "sweet news" for the family.

"The family has eyes only to see him again. We are all waiting for him," Khan said in a statement to Reprieve.

"He has spent more than 14 years of his life in this dangerous and terrible prison without charge. We are thankful to the judges and to everyone who continue to press for his freedom."

Still, despite his detention having no legal basis and him being cleared for release, it could take some time before Gul leaves the prison. More than a decade ago, 17 Uyghur men were ordered free, however, they remained at Guantanamo for several years while the US government worked to find countries in which to resettle them.

Of the 39 men currently detained at the prison, 12 have been approved for transfer. So far under the Biden administration, only one detainee has been released - Moroccan national Abdul Latif Nasser.

According to the New York Times, another lawyer for Gul, Eric Lewis, urged the State Department to appoint a special envoy to coordinate the transfers of detainees from Guantanamo Bay.

Any men cleared for transfer should be sent "home or to countries that are willing to assist", he said in a statement.

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