Civilian staff at GCHQ risk being prosecuted for war crimes as a result of a legal action being launched tomorrow over the alleged use of British intelligence in the CIA's "targeted killing" programme.

Human rights lawyers will issue proceedings saying that employees at the UK intelligence agency who assist the US in directing drone attacks in Pakistan could be liable as "secondary parties to murder" and that any UK guidance allowing the passing of information to the CIA for use in the strikes is unlawful.

Pakistan has previously condemned the attacks as a violation of its sovereignty, amid concern that the use of US drones contravenes international humanitarian law. Hundreds of innocent civilians are thought to have been killed as a result of drone attacks.

Questions are also mounting over the role of British officials in assisting the CIA's targeting of alleged militants in Pakistan. Reports suggest that GCHQ, the intelligence agency for which the foreign secretary, William Hague, is responsible, provides "locational intelligence" to the US.

The legal action, brought by the law firm Leigh Day & Co and the legal action charity Reprieve, is directed against Hague on behalf of Noor Khan, whose father was killed in a US drone strike in Pakistan last year.

Malik Daud Khan was presiding over a peaceful council of tribal elders in the North Waziristan tribal area when a missile was fired from a drone, believed to have been CIA-operated. Khan was one of more than 40 people killed.

The attack has intensified scrutiny over the levels of complicity between the UK and US over drone strikes. Reports have quoted GCHQ sources as justifying their forwarding of intelligence to the US as being in "strict accordance" with the law, a claim contested by lawyers.

The issuing of legal proceedings at the high court tomorrow challenges the lawfulness of such alleged complicity, arguing that "there is also a significant risk that GCHQ officers may be guilty of conduct ancillary to crimes against humanity and/or war crimes, both of which are statutory offences under the International Criminal Court Act, 2001".

Only individuals entitled to immunity from ordinary criminal law in respect of armed attacks are considered under international law as "lawful combatants" participating in an "international armed conflict", according to the legal papers.

Richard Stein, the head of human rights at Leigh Day & Co, said that staff at GCHQ are civilians, not combatants, and that there is no recognised "international armed conflict" in Pakistan.

He added: "We believe that there is credible, unchallenged evidence that the secretary of state 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.

"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 and/or the lawfulness of such attacks; and his policy, if implemented, involves the commission of serious criminal offences by employees of GCHQ or by other officials or agents of the UK government in the UK."

Lawyers want to discover if there is any UK policy or guidance dealing with the circumstances in which information possibly used in directing drone attacks in Pakistan can be shared with the US.

Clive Stafford Smith, director of legal action charity Reprieve, said: "What has the government got to hide? If they're not supplying information as part of the CIA's illegal drone war, why not tell us? And if they are, they need to come clean."

American officials have privately admitted that the CIA's drone programme has killed many Taliban and al-Qaida commanders. A Foreign Office spokesman said that he could not comment on the issue.