The Ministry of Defence has been asked to reveal whether the UK had a role in supplying information to a US military "kill list" in Afghanistan.
An Afghan man who lost five relatives in a missile strike in 2010 has started legal proceedings against the MoD and the Serious Organised Crime Agency.
He wants to know the extent of UK involvement in the joint integrated prioritised target list (JIPTL).
The MoD says it works "strictly within the bounds of international law".
The list contains names of people which military forces in Afghanistan consider to be targets.
Solicitors Leigh Day & Co have written to Defence Secretary Phillip Hammond and Soca asking them to provide assurances over Britain's participation in the compilation of the list, including whether it complies with UK and international law.
'Precision air strike'
The legal challenge is being brought by Habib Rahman, who lost two brothers, two uncles and his father-in-law in a missile attack on 2 September 2010.
The attack happened while they were helping his cousin Abdul Wahab Khorasani, a former parliamentary candidate, as he campaigned in the Rustaq district of Takhar province, Afghanistan.
Mr Rahman said 10 civilians were killed and several more injured, most of whom were election campaign workers and relatives.
At the time, the International Security Assistance Force (Isaf) said it was "a precision air strike" which killed or injured "eight to 12 insurgents", including a Taliban commander.
But lawyers say there is "powerful evidence" that the attack was a case of mistaken targeting, following a detailed investigation carried out by Kate Clark of the Afghanistan Analysts Network.
They say the details of the attack suggest the "kill list" does not comply with international humanitarian law and the criteria to decide which individuals are included breaches the Geneva Conventions.
Mr Rahman's lawyers say they do not know whether information provided by the UK was involved in the attack.
But they say they hope the challenge will force officials to be more open about the UK contribution to the "kill list".
Rosa Curling, from Leigh Day & Co, said: "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.
"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."
An MoD spokesman said: "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 it worked 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."
The JIPTL was described as a "kill list" in a report to the US Senate's committee on foreign relations.
It 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".