A Pakistani man who claims he was illegally detained and tortured by British forces in Iraq has won the right to sue the UK government.

The legal system would be "failing in its duty" if it did not deal with the claims of Yunus Rahmatullah, the judge in the case told the courtroom.

Rahmatullah was allegedly tortured by UK forces in Iraq in 2004 before he was handed over to US custody. The US reportedly transferred him to Afghanistan via the notorious Abu Ghraib prison in Baghdad. Rahmatullah claims he was secretly detained and tortured by the US for nearly a decade before his release in June of this year.

The UK denied any involvement in Rahmatullah's rendition until 2008, when then-Defense Secretary John Hutton publicly admitted that it occurred.

In a statement to VICE News, Nick Mercer, the former chief legal advisor to the UK armed forces in Iraq in 2003, said the judgement was "long overdue," and could potentially "remove the cloak of secrecy" surrounding the case.

"The judgement in the case of Yunus Rahmatullah is to be warmly welcomed," Mercer said. "The UK protests its innocence in public whilst hiding behind the doctrine of State immunity in court."

Yunus Rahmatullah in an undated photo provided by his family. (Photo via Associated Press)

The ruling comes soon after a Court of Appeal ruling that a separate rendition case involving Abdul-Hakim Belhaj should be heard in the British courts.

In 2011, the 48-year-old Belhaj attempted to sue the UK government over claims he was illegally rendered to Libya and tortured. His claim was struck down in 2013, but last month the Court of Appeal ruled that the case could go ahead.

Mercer also noted that the UK's Iraq Historic Allegations Team (IHAT) is still refusing to investigate other allegations of rendition by the UK in Iraq.

"Rendition is a potential war crime, yet no authority in the UK seems to want to investigate such matters despite instances being brought to their attention," Mercer said. "It is hoped that judgements such as these and the intervention of the ICC will ensure proper investigations and the eventual compliance of the UK with International Humanitarian Law."

Kat Craig, legal director at Reprieve and part of the legal team for Rahmatullah, told VICE News that, "the Government has tried everything to avoid coming to court," to explain Rahmatullah's ordeal, "but this week's judgment blows their excuses out of the water."

"No more delays," Craig said. "Yunus and other victims of British rendition and torture must have their day in court, and lessons must be learned so that these horrific abuses never take place again."