Defence chiefs are facing a legal challenge to provide details of Britain's role in supplying information to an American military "kill list" in Afghanistan.
An Afghan man who lost five relatives in a missile strike has started proceedings against the Serious Organised Crime Agency (Soca) and the Ministry of Defence demanding to know the UK's involvement in the joint integrated prioritised target list (JIPTL).
Solicitors Leigh Day & Co have asked Defence Secretary Philip Hammond and Soca to provide a series of assurances about Britain's participation, including guidance on whether it complies with UK and international law.
The list includes names of people that military forces in Afghanistan have designated as targets.
A report to the US Senate's committee on foreign relations described the JIPTL as a "kill list" and stated: "The military places no restrictions on the use of force with these selected targets, which means they can be killed or captured on the battlefield".
Habib Rahman, who has brought the legal challenge, lost two brothers, two uncles and his father-in-law in a missile attack on September 2 2010.
The attack happened while they were helping Mr Rahman's cousin Abdul Wahab Khorasani, a former parliamentary candidate, as he campaigned in the Rustaq district of Takhar province, Afghanistan.
Ten civilians were killed and several more injured, most of whom were election campaign workers and relatives of Mr Rahman, he said.
The International Security Assistance Force (ISAF) said at the time the attack was "a precision air strike" which killed or injured "eight to 12 insurgents", including a Taliban commander.
However lawyers say a detailed investigation carried out by Kate Clark of the Afghanistan Analysts Network provides "powerful evidence" that the attack was an instance of mistaken targeting.
Leigh Day & Co claim the details of the attack suggest the "kill list" is not compliant with international humanitarian law and the criteria to decide which individuals are included breaches the Geneva Conventions.
Rahman's lawyers have acknowledged they do not know whether information provided by Britain contributed to the attack which killed Mr Rahman's relatives.
However they said they hoped the legal challenge will force officials to be more open about the British contribution to the "kill list".
Rosa Curling, from Leigh Day & Co, said: "At a time when the UK government is arguing that our presence in Afghanistan is needed to bring stability to the country and to establish and maintain the rule of law, ensuring that the UK government and its agencies themselves are operating within their legal obligations could not be more important.
"Our client's case suggests that the establishment and maintenance of the 'killing list' is not in line with the UK's duties under international humanitarian law. Our client lost five of his relatives in an attack by the international military forces as a result of this list.
"It is important that the MoD and Soca provide us with the reassurances sought, to make sure that others do not suffer the tragic loss of life as experienced by Mr Rahman."
The MoD said it could not discuss the case in detail but stressed it worked "strictly within the bounds of international law".
An MoD spokesman said: "As part of the International Security Assistance Force (ISAF), UK Forces operate alongside numerous partner nations in the fight against an insurgency that seeks to maim and kill both innocent Afghan civilians and allied forces alike.
"We continue to work towards a stable Afghanistan that can look after its own security by the time our combat operations cease at the end of 2014.
"In doing so, UK Forces operate strictly within the bounds of international law under rules of engagement which, for reasons of operational security, we do not discuss in detail."
A Soca spokesman said: "Soca does not discuss intelligence. Soca works strictly within the bounds of international law.
"Our activity overseas is conducted in line with other UK Government departments, which comply with the principles of international humanitarian law and human rights."