Today, as on 14 February every year, couples across the world celebrate Valentine's Day. But for Yunus Rahmatullah, the date this year marks a far lonelier occasion.

Yunus is just one of some 3,000 men, women and children who have spent years trapped in America's prison in Bagram, Afghanistan. Like many others, he has never been charged with any crime. But, unlike many less fortunate than him, the British courts have actually ordered he must be released. If he is not, the UK government stands in breach of the Geneva Convention: and today is the deadline for its response.

Not guilty of any crime

Yunus, also known by his nickname "Saleh Huddin", was captured by British forces in Iraq in 2004 and handed over to the Americans for transfer to the notorious Bagram air base, where countless other "war on terror" suspects disappeared during that decade. For six years his family had no idea where he was, and as a result of his brutal treatment in UK and US custody, he is now in "catastrophic mental and physical shape".

Thanks to the work of the Red Cross in identifying hundreds of Bagram prisoners, and then Reprieve, who took up Yunus's case, the US authorities were finally forced to admit he is not guilty of any crime, and cleared him for release 18 months ago.

Yet somehow he, along with countless other prisoners cleared for release, still languishes in jail.

Why has it taken so long?

In many cases, the hold-up seems to be on both sides: on the one hand, the US administration doesn't wish to look soft on terror; on the other, national governments – particularly of Pakistan, Afghanistan, and several Gulf states – don't wish to admit their complicity in the so-called "war on terror" by taking back the citizens they left in American custody for so long. As a result, many of the prisoners remain in legal and diplomatic limbo.

But in Yunus's case, no such ambiguities exist. In December, Britain's court of appeal ruled that the UK authorities (his original captors) must secure his release from the Americans, or stand in breach of the Geneva Convention, leaving British officials open to prosecution for war crimes. The UK government has duly written to their US counterparts asking for his release.

As yet, there has been no official reply – and the deadline set by the court to secure his release expires today.

When Yunus's mother, Fatima, finally found out what had happened to her son, she wrote: “Yunus is the youngest and closest son to my heart. I lost my other son, his only brother, in a tragic accident. Now, Yunus is my only hope in life. I see him in my dreams; I pray daily that I will see him in my waking hours again."

Take action: Donate to support Reprieve's legal campaign for the release of Bagram detainees, and other prisoners unlawfully held across the world.