A British law firm said it was to sue Foreign Secretary William Hague on behalf of a Pakistani man over claims that British intelligence was used to assist US drone attacks.

London-based Leigh Day and Co confirmed they would issue formal proceedings at Britain's High Court on behalf of Noor Khan, whose father was killed by a US strike in Pakistan.

Lawyers will claim that civilian intelligence officers who pass on intelligence to the US are not "lawful combatants", therefore cannot claim immunity from criminal law and could be liable as "secondary parties to murder".

They will also argue that the immunity clause does not apply as Pakistan is not currently involved in an "international armed conflict".

"There is credible, unchallenged evidence that (Hague) is operating a policy of passing intelligence to officials or agents of the US government and that he considers such a policy to be in 'strict accordance' with the law," Richard Stein, head of human rights at Leigh Day, said in a statement.

"If this is the case, the Secretary of State has misunderstood one or more of the principles of international law governing immunity for those involved in armed attacks on behalf of a state."

Britain's Foreign Office said it would not comment on legal or intelligence matters.

Khan says his father, Malik Daud, was killed by a drone missile while at a council of elders meeting in northwest Pakistan.

Drone attacks have become a key feature of US President Barack Obama's fight against terrorism in Pakistan, but many inhabitants are deeply unhappy about the civilian death toll incurred in the raids.