Two Government departments have refused to publish the legal advice that paved the way for Britain to launch a drone strike on Islamic State targets in Syria.
The Attorney General's Office and Cabinet Office rejected a freedom of information (FoI) request, saying it was in the public interest for the details to remain confidential.
Last month David Cameron disclosed that an RAF drone had killed two Britons in an attack near the city of Raqqa, describing it as an "act of self defence".
Reyaad Khan, from Cardiff, the primary target, and Ruhul Amin, from Aberdeen, died on August 21.
Announcing the operation, the Prime Minister said it was "entirely lawful" and confirmed that the Government's senior law officer, Attorney General Jeremy Wright, was consulted and "was clear there would be a clear legal basis for action in international law".
Ministers have resisted calls for the full advice to be published and this position was repeated in correspondence to the Press Association.
The Cabinet Office and Attorney General's office were asked for details of the legal advice provided to Mr Cameron and the National Security Council before the strike was authorised.
In near identical replies, they confirmed they hold information "falling within the scope of your request". However, they declined to provide the material, citing six exemptions built into the FoI Act.
They were: information supplied by, or relating to, bodies dealing with security matters; defence; international relations; information relating to the provision of advice by any of the law officers or any request for the provision of such advice; protection of personal data; and legal professional privilege.
The exemptions on security and personal data are "absolute" while the others are "qualified", meaning officials must weigh up whether the public interest in maintaining them outweighs the public interest in disclosing the information.
The Attorney General's Office and the Cabinet Office said in their replies that they "recognise that there is a public interest in demonstrating to the public that the Government sought and received appropriate professional legal advice, and that this military action took place in accordance with the rule of law".
However, they said: "The public interest in maintaining legal professional privilege, and in particular in maintaining the confidentiality of legal advice provided by the law officers, is of particular importance in this case.
"There is a strong public interest in the Prime Minister, Secretary of State for Defence and Government more broadly being able to seek legal advice in confidence.
"The particular importance of maintaining the confidentiality of advice given by the law officers is reflected in the convention, observed by successive governments, that their advice should not be disclosed outside government."
The departments added: "This strong public interest in government being able to seek legal advice in confidence is heightened in the context of matters of the importance and sensitivity of those in issue here - engaging matters of intelligence, defence, national security and foreign relations - in respect of which it is exceptionally important that the government should be able to ask its most senior legal adviser for full and careful advice, with confidence that it will remain confidential."