The detention policy adopted by the UK's armed forces in Afghanistan is unlawful, the High Court has ruled.
Justice George Leggatt examined the case of Serdar Mohammed, an Afghan farmer who was held without charge for over three months.
He found that the farmer was fairly captured and held for four days but his continued imprisonment in UK military bases for 106 more days violated Afghan, British and international law.
Mr Mohammed was arrested by British troops on suspicion of being a Taliban commander in Helmand province in April 2010.
He claimed he was not allowed access to a lawyer while being held without charge and was brutally tortured into giving a false confession by Afghan security services at Lashkar Gah after being handed over.
Mr Mohammed filed a damages claim for assault, false imprisonment and human rights breaches but his allegations, particularly those of ill-treatment at the hands of UK armed forces, are strongly disputed.
Philip Hammond, the Secretary of State for Defence, wanted to contest them at a full trial but Mr Mohammed applied for preliminary issues relating to the lawfulness of his detention from April to July 2010 to be determined.
The judge said that the International Security Assistance Force (Isaf) had operating procedures permitting detention for a maximum of 96 hours, after which a person must be released or given into the custody of the Afghan authorities.
UK armed forces adhered to this policy until November 2009, when the Government adopted its own national rules where ministers could authorise imprisonment beyond 96 hours for the purpose of interrogating a detainee who could provide significant new intelligence.
Justice Leggatt said the Human Rights Act only extended to Mr Mohammed’s arrest and initial detention.
He added: “Nor was it compatible with Article 5 (of the Human Rights Act) to detain him for a further 25 days solely for the purposes of interrogation and without bringing him before a judge or giving him any opportunity to challenge the lawfulness of his detention.
"Mohammed's continued detention by the UK for another 81 days for 'logistical' reasons until space became available in an Afghan prison was also unlawful for similar reasons and was not authorised by the UN Security Council.
“In addition, this further period of detention was arbitrary because it was indefinite and not in accordance with the UK's own policy guidelines on detention."
Three other Afghan nationals- Mohammed Qasim, Mohammed Nazim and Abdullah (who has no surname) – have also brought claims about their detention and the ruling could have far-reaching consequences.
General Sir Nicholas Houghton, the chief of defence staff, said the judgement caused operational concerns and applied “peace time” laws out of context.
He added: “Our freedom to conduct detention operations and to exploit detainees for intelligence is vital to our ability to protect the lives of innocent civilians and our own forces.
“Any judgment which might inhibit our ability to operate in ways which minimise casualties and protect our people is deeply worrying.”
Mr Hammond, the Defence Secretary, said the Government would appeal the judgement and look at other options if unsuccessful.
“We cannot send our Armed Forces into battle with both hands tied behind their backs. Our troops engaged in operations must be able to detain our enemies who aim to maim and kill UK service personnel and innocent civilians,” he added.
“It cannot be right for the European Convention on Human Rights to apply on the battlefield, restricting the ability of our troops to operate in combat.”
Mr Mohammed cannot recover damages in English courts because his imprisonment by UK forces was illegal under Afghan law but the Convention gives him an ”enforceable right to compensation“.
Justice Leggatt said the MoD should not have been surprised by the decision as there was “no legal basis” for detention beyond Isaf rules.
He said: "UK ministers nevertheless decided to adopt a detention policy and practices which went beyond the legal powers available to the UK. The consequence of those decisions is that the MOD has incurred liabilities to those who have been unlawfully detained."
British forces joined the US army and Nato allies in Afghanistan in 2001 to fight al-Qaeda and remove the Taliban from power following the September 11 attacks.
A helicopter crash last month took the number of British casualties in the conflict to 453 and violence has increased as the withdrawal of foreign forces continues.
A spokesman for law firm Leigh Day, which represented Mr Mohammed, said: “When we send our troops abroad it is the Ministry of Defence’s job to ensure that the mechanisms are put in place to ensure that they operate within the rule of law.
“As the Court has made clear, the MoD fully understood the parameters of the law regarding detention and yet they decided to operate flagrantly outside those rules.”
Additional reporting by PA