By Dominic Casciani
Binyam Mohamed is among the men behind the damages claim
Six former Guantanamo Bay detainees have overturned a ruling that allowed government use of secret evidence to defend itself against a damages claim.
The men had been told that parts of MI5 and MI6's defence could be kept secret.
The men are suing the UK government, saying their detention by the US could have been stopped.
The Court of Appeal ruled it would "take a stand" against secrecy that would undermine the "most fundamental principles of common law".
The detainees were held in foreign prisons at the instigation of US forces. They ended up at Guantanamo Bay in Cuba before the British government negotiated their return to the UK.
But the men are now suing MI5, MI6, the Foreign Office, the Home Office and attorney general saying that they played a part in, or failed to stop, their detention and ill-treatment.
Previously, government lawyers had successfully argued at the High Court that parts of a defence against any damages claim, where the allegations involved national security issues, should be kept from the public.
But in a strongly-worded ruling, the Master of the Rolls Lord Neuberger, sitting with two other senior Court of Appeal judges, said that no damages hearing could be heard in secret because the courts had not been empowered by Parliament to withhold evidence from the claimants.
"In our view, the principle that a litigant should be able to see and hear all the evidence which is seen and heard by a court determining his case is so fundamental, so embedded in the common law, that, in the absence of parliamentary authority, no judge should override it," said Lord Neuberger.
"At least so far as the common law is concerned, we would accept the submission that this principle represents an irreducible minimum requirement of an ordinary civil trial.
"Unlike principles such as open justice, or the right to disclosure of relevant documents, a litigant's right to know the case against him and to know the reasons why he has lost or won is fundamental to the notion of a fair trial."
The ruling is the second major blow by the Court of Appeal against the government's attempts to keep secret material out of open courts.
Earlier this year, the court revealed a summary of what UK intelligence officials knew of the ill-treatment of one of the claimants, Binyam Mohamed.
British courts deal with secret evidence in two different ways. Ministers can sign special orders called Public Interest Immunity certificates, which leads to material being withdrawn entirely from a case so it cannot be used by either party.
Jamil el Banna
Control order hearings and national-security related deportations hear some evidence in secret, such as MI5 assessments. The suspect is not allowed into these hearings - but a special advocate argues on their behalf behind closed doors.
The government said this second procedure should be permitted in the six men's damages claim, saying national security could be protected while still allowing someone to argue the men's corner.
But in their judgement, Lord Neuberger, Lord Justice Maurice Kay and Lord Justice Sullivan said senior judges had to "take a clear stand".
"Quite apart from the fact that the issue is one of principle, it is a melancholy truth that a procedure or approach which is sanctioned by a court expressly on the basis that it is applicable only in exceptional circumstances nonetheless often becomes common practice," said Lord Neuberger.
A spokesman for the government said that it was considering the judgement.
With ministers away for the election campaign, it isn't clear whether the government will attempt to have the case heard by the Supreme Court.
Corinna Ferguson, a lawyer at campaign group Liberty, said: "Yet again, the Court of Appeal has sent the strongest signal to the security establishment that it cannot play fast and loose with the rule of law.
"Fair and open justice belongs to people not governments. Whoever governs us from Friday would be wise to bear this in mind."