The government’s power to block requests for information on national security grounds has been significantly curtailed by a tribunal ruling over targeted killings of British jihadists abroad.

Although attempts to obtain the legal advice given to the prime minister before RAF drone strikes in Syria in 2015 were dismissed, the court said officials could not rely on a blanket ban preventing the release of all relevant details.

The upper tribunal administrative appeals chamber decision, handed down shortly before Christmas, said freedom of information requests in such sensitive policy areas should be subject instead to qualified exemptions in which security concerns are balanced against wider public interests.

The tribunal also criticised the Information Commissioner’s Office for accepting assurances by government departments that the material was all exempt from FOI applications. The ICO, it emerged, had not even read the documents in the case.

The challenge was brought by Rights Watch (UK) after RAF drone strikes against two British citizens, Reyaad Khan and Ruhul Amin, which prompted accusations that officials were operating a US-style “kill list”.

At the time, the prime minister, David Cameron, described the attack as a “new departure” and said he had relied on legal advice from the attorney general, Jeremy Wright QC, which assured him it was “entirely lawful”. Later that year, UK and US forces cooperated on an airstrike that killed Mohammed Emwazi, nicknamed “Jihadi John”, the Islamic State extremist held responsible for killing several western hostages.

The day after the attack on Khan and Amin, Rights Watch (UK) submitted a freedom of information request for the legal advice, or a summary of it, to be published.

Since then, Wright has given some further details about the legal context in which such decisions are made.

“Specific” advance evidence of a terror plot threatening UK interests is not legally necessary before launching pre-emptive drone strikes against suspects overseas, Wright said last year.

The legal threshold for self-defence was not being “watered down”, he stressed, adding that not knowing the target, type or time of a terrorist attack should not prevent military action.

In the upper tribunal hearing, the government argued that section 23 of the Freedom of Information Act 2000 provided an absolute exemption on handing over any information passing through the security agencies.

The upper tribunal rejected that argument, saying any advice should be “disaggregated” and considered under other exemptions provided for by the act.

It declared: “Although we accept that the disaggregated information … can be said to relate to [the security agencies], parliament did not intend such information to be covered by the absolute section 23 exemption.”

Parts of the legal advice, such as the government’s interpretation of international law, should be subject to a public interest test, the tribunal said.

Yasmine Ahmed, the executive director of Rights Watch (UK), said: “This is a significant pushback against the government’s expansive claims of secrecy that would have allowed it to claim absolute secrecy and suppress information that had merely passed through the hands of the security services, corroding the public’s right to information.

“The government [is] on notice: referencing the security services or them having sight of information does not hand public bodies a blank cheque to veto scrutiny of their actions, and they cannot do so and expect the information commissioner, courts and public to acquiesce.”

Daniel Carey, a solicitor at the law firm Deighton Pierce Glynn, which represented Rights Watch (UK), said the challenge “has been vindicated by these significant rulings on the legal and procedural approach to security services information under the Freedom of Information Act”.