Campaigners appeal against ruling that government can keep advice secret because it relates to security agencies

The UK government’s refusal to reveal legal advice justifying targeted killings of British jihadists abroad will be challenged at an appeals tribunals on Thursday.

The case is brought by Rights Watch UK following RAF drone strikes in 2015 against two British citizens, Reyaad Khan and Ruhul Amin, in Syria which prompted accusations that officials are operating a US-style “kill list”.

Critics have accused the government of undermining international law and lowering the permissible threshold for pre-emptive, global military strikes.

At the time, the prime minister, David Cameron, described the attack as a “new departure” and said he relied on legal advice from the attorney general, Jeremy Wright QC, which assured him that it was “entirely lawful”.

The government pledged to carry out further targeted killings, including in countries with which the UK was not at war. Later that year, UK and US forces cooperated on an airstrike that killed the Islamic State executioner Mohammed Emwazi, nicknamed “Jihadi John”.

The day after the attack on Khan and Amin, Rights Watch submitted a freedom of information request for the legal advice, or a summary of it, to be published. Disclosure was a matter of “acute public importance”, the civil liberties organisation said, and parliament needed to have sufficient information to debate the radical policy development.

Both the attorney general and the Cabinet Office rejected the request. The Cabinet Office said the entire legal advice was exempt from release under section 23 of the Freedom of Information Act 2000 because it related to security agencies. The attorney general’s office initially said only part of the advice was withheld for this reason, according to Rights Watch.

The refusal to publish was subsequently upheld by the Information Commissioner’s Office. Rights Watch is now appealing against the ICO’s decision. It argues that the government is required to “disaggregate” the legal advice: those parts relating to security bodies can legitimately be kept secret, while other sections, such as interpretation of international law and application of legal principles, should be disclosed.

Yasmine Ahmed, executive director of Rights Watch, said: “This unprecedented strike signalled a profound policy shift and a sweeping expansion of the circumstances in which the government claims they can lawfully use force overseas. By shielding their legal justification from public view the government are depriving parliament and the public of the opportunity to have informed debate about where and when its government can engage in the premeditated killings of individuals, including British nationals, abroad.

“The government is seeking to have its cake and eat it: explicitly relying on the legal advice and its conclusions to assure parliament this new departure was lawful, then denying parliament and the public the opportunity to properly test this claim by wholesale withholding the advice with a catch-all reference to security matters.”

Daniel Carey, a solicitor at the law firm Deighton Pierce Glynn, which is representing Rights Watch, said: “It is important to know what the legal advice that enabled the strike – as opposed to the justifications that followed – was.

“We are arguing that the presence of intelligence-related material in the attorney general’s advice should not prevent … the British public knowing even the basic approach that was taken to key questions of international law governing the use of force on foreign territory. Redaction of the document is likely to be necessary, but we are asking the tribunal to rule that outright refusal is not justified.”

A key issue is whether Khan posed an “imminent” threat – a legal requirement to trigger the right to self-defence under article 51 of the United Nations charter.

Earlier this year the attorney general gave a speech explaining the dilemma in which he said “specific” advance evidence of a terror plot threatening UK interests was not legally necessary before launching pre-emptive drone strikes against suspects overseas.

Addressing the International Institute for Strategic Studies, Wright denied that the threshold for self-defence was being “watered down” but said not knowing the target, type or time of a terrorist attack should not prevent military action.

Parliament’s joint committee on human rights (JCHR) and the intelligence and security committee (ISC) both instigated inquiries in the wake of the strike. The JCHR said it was “disappointed by the government’s unhelpfulness” in refusing to provide it with a detailed memorandum on its legal position. The ISC report expressed concern about the failure of Downing Street to allow access to “what we consider to be relevant documents”.

The hearing is expected to last two days.