Public interest immunity (PII), previously known as Crown privilege, is a principle of English common law under which the English courts can grant a court order allowing one litigant to refrain from disclosing evidence to the other litigants where disclosure would be damaging to the public interest.
In Conway v Rimmer [1968], the House of Lords held that the courts retained the final decision on whether PII should be upheld, and, in R v Chief Constable of West Midlands, ex parte Wiley [1995], the House of Lords decided that a minister could discharge his duty by making his own judgment of where the public interest lies (that is, to disclose or to assert PII).
However, PII is not limited to the Crown (see the NSPCC case mentioned below), and cannot be waived save in exceptional circumstances.
A number of PII certificates were signed in relation to the prosecutions of individuals involved in the Matrix Churchill "Arms to Iraq" case, a subject that was subsequently investigated in the Scott Report.
For example, MI6 have not succeeded in obtaining a PII certificate since the 1995 Tomlinson case, and have thus been subject to court scrutiny for investigations such as the inquest into the death of the Princess of Wales, and allegations that their officers partook in torture.