It seems likely that the US military will soon seek to prosecute Binyam Mohamed in a military tribunal that fails to meet internationally recognised legal standards.

British officials have condemned the tribunals, and the former lord justice Steyn referred to them as "kangaroo courts". The British Government has strongly opposed Mr Mohamed being put on trial in such an unfair process, and has demanded that he be returned home to London.

This is admirable. Why, then, does the Government refuse to provide Mr Mohamed with assistance in proving his claim that he is innocent, and that he was tortured? Some magnet seems to have dragged the moral compass of this government radically off course.

With the generous assistance of the law firm Leigh Day, Mr Mohamed recently sued the British Government, asking simply that it provide evidence in its possession that would help us prove that Mr Mohamed was rendered by the CIA to Morocco, where he underwent 18 months of torture, including having a razor blade taken to his genitals. The Government's lawyers replied that "evidence held by the UK Government that US and Moroccan authorities engaged in torture or rendition cannot be obtained" by Mr Mohamed's lawyers to help defend him. Why not? We are not told.

The government lawyers went on to write that "the UK is under no obligation under international law to assist foreign courts or tribunals in ensuring that torture evidence is not admitted" against Mr Mohamed in Guantanamo. Again, why not?

"The trouble with the law is lawyers," wrote the famous American lawyer Clarence Darrow. No matter what the technical arguments that can be made in favour of an injustice, surely the just course of action is obvious here. We do not ask anyone merely to accept Mr Mohamed's claims, but we do ask that he be given an open trial in which to present them to a fair jury, and that he be allowed access to the evidence with which to prove his case.

We know that the British Government has various pieces of helpful evidence in its possession. For example, British agents met with Mr Mohamed when he was arrested in Pakistan, interviewed him for three hours, and apparently told the US that he was a "nobody" (a janitor from Kensington) as well as telling Mr Mohamed that he was going to be rendered by the US to a foreign country. When Mr Mohamed was duly sent to Morocco, the UK provided background information on him to the US that was used to manipulate him as part of his torture.

Why would the British Government refuse to disclose this kind of critical information? Here, we are left to speculate, since they won't give an explanation. Sadly, the most likely reason is that there is much more evidence that they would rather remained hidden – such as proof of systematic British co-operation in the US rendition process.

The British citizen used to have an absolute right to remain silent in the face of a criminal accusation. Two decades ago, this right was abolished. Surely, no official can be allowed to remain silent when asked to explain why the Government wants to cover up evidence that could help bring justice to a Londoner held in Guantanamo Bay?

The author is the director of Reprieve, the legal charity for the human rights of prisoners

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