Public interest immunity

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Public interest immunity (PII) is a rule in English law that lets a court stop one side in a case from sharing certain evidence because showing it would harm the public interest. It is an exception to the usual rule that all relevant evidence must be disclosed. The court has to balance two public interests: ensuring fair justice by providing relevant material, and protecting information whose disclosure could cause damage.

PII is often used to protect official secrets or the safety of informants in serious crime cases. When the government wants PII, a minister signs a PII certificate, and the court then decides whether the information should be disclosed or kept secret. Usually the court does not examine the documents itself; it will only look at them if there is doubt about whether PII applies.

Historically, PII used to be called Crown privilege and came from the idea that the Crown had special immunity. It is not limited to the Crown and is hard to waive except in exceptional circumstances. Key legal cases shaped how PII works: Conway v Rimmer (1968) said the courts have the final say on PII, while R v Chief Constable of West Midlands ex parte Wiley (1995) held that a minister can decide, in practice reducing how often PII is used.

The right to a fair trial under the European Convention on Human Rights supports that both sides should have access to the same evidence, but this right is not absolute. Public interest can override it if it is strictly necessary to protect a bigger public good.

In recent years, far fewer PII certificates are issued. For example, MI6 has not succeeded with PII since 1995. PII has appeared in high-profile cases such as investigations into Princess Diana’s death, allegations of torture, and the Matrix Churchill “Arms to Iraq” case, which led to the Scott Report.