1. Since a House of Lords decision in 1968, the Crown's privilege to prevent disclosure of documents in legal proceedings has become a right (no longer restricted to the Crown) to claim non-disclosure in the public interest. The decision whether to accept or reject the claim is taken by the court, if necessary after inspecting the documents in question.
2. In reaching its decision, the court is required to balance two considerations: the interest of the proper functioning of the public service, and the interest of the proper administration of justice.
3. It follows that the Crown (ie, the relevant minister) is under no legal duty to claim non-disclosure. He or she has a power to do so, having regard to his or her view of the relevance of the above considerations to the particular documents.
4. Ministers may consider themselves under some sort of non-legal obligation to claim non-disclosure in certain kinds of cases (eg, as regards certain classes of documents or affecting certain kinds of governmental activity). But that is a matter of self-constraining behaviour, not a matter of legal duty.
5. The arguments put forward by a minister claiming public interest immunity are at the discretion of the minister. A claim in a particular case may be the subject of political accountability to Parliament. False or misleading statements might be in contempt of court or might conceivably give rise to criminal or civil liability.