Today’s story about the move by the Home Secretary to keep a forthcoming murder trial secret casts a spotlight on public interest immunity — a principle of English common law developed by judges over the centuries.

Government ministers sometimes seek public interest immunity, or PII certificates, in trials to protect official secrets — and so they are often described as “gagging orders” although Sir Nicholas Lyell, QC, when Attorney-General, condemned the phrase as “highly misleading”.

The most famous recent case in which PII certificates were signed was in the prosecution of individuals involved in the Matrix Churchill Arms to Iraq case. The subsequent inquiry under Sir Richard Scott (now the law lord Lord Scott of Foscote) found in its report in 1996 that PII certificates had been issued to keep from defence counsel documents that would have exonerated the defendants on trial.

Public interest immunity used to be called Crown privilege and stems from the same principle as the immunity of the Crown from prosecution in the Crown Proceedings Act 1947. But PII is not limit to the Crown. PII is claimed in both criminal and civil trials when one party seeks an order to stop material being disclosed to the other side.

Several principles come into play: that of the proper administration of justice along with the importance of open justice — set against national security and individual safety. PII usually arises arises during pre-trial disclosure of all the evidence. In a criminal trial, if the prosecution believes that disclosure of material will put at risk an individual such as a police informant or agent or be damaging to the public interest, then PII may be claimed. As well as material relating to the work of the intelligence services, the immunity may be claimed where evidence has been gathered in confidence, such as in child cruelty cases, relates or comes from police informants or undercover work or discloses police or covert surveillance techniques.

Crucially, in the wake of a landmark Lords’ ruling in 1968, the issue must then come before a judge to decide if the certificate can stand and the documents be withheld or that the documents are central to a case but the hearing must be in private. The Lords’ ruling stated that a court was no longer bound to accept an assertion of PII by a minister — judges could make their own independent judgment as to what the public interest required.

In 2002 at the trial of Paul Burrell, the butler to Diana, the late Princess of Wales, the prosecution applied to the judge for a ruling that disclosure of certain information would be harmful to the public interest and should not be made public.

The move came after the Queen’s disclosure that she had remembered a conversation she had had with Mr Burrell in which the former butler told her that he had taken items from the late Princess’s home for safekeeping.

In response the prosecution sought to have the information covered by PII. No ruling was made: instead, the trial judge, Mrs Justice Rafferty, said that the PII issue had “been resolved” and the Crown announced charges were to be withdrawn.

Also in 2002, a PII certificate was presented to the court by the Crown Prosecution Service just ten minutes into the hearing in a case brought against the Cornish Stannary Parliament. The Stannary Parliament suggested that the reason was that Duchy of Cornwall had refused to reveal the circumstances in which it transferred several its properties, including Tintagel Castle, to the care of English Heritage.

In another case in 1983, a group of airlines claimed that the British Airports Authority had unlawfully increased landing fees at the instigation of a government minister. The minister disclosed some documents but claimed PII over others. The House of Lords ruled that it did not need to inspect the documents, saying that inspection was required only if the documents were “reasonably likely” to assist or damage a party’s case.

The test is whether damage will be caused to the public interest or to individuals, putting them at risk. Professor Gary Slapper, Director of the Open University Law Programme, said that the basic principles of safeguarding of the public interest was a duty of organisations such as the police and could not be waived by a party or witness who wanted disclosure of sensitive material if the public interest requires it to be excluded.

What if there is a dispute? Then, he says, the judge decides whether exclusion is required after weighing all competing considerations in the balance. “In criminal cases, one of the most important considerations is that, if the material in question, such as the identity of key witnesses, tends to raise any doubt as to the guilt of a defendant, it may be impossible to proceed with the trial unless that evidence is disclosed.”

The judge who must decide whether to allow the PII in the forthcoming trial of Wang Yam, a financial trader from Hampstead, for the murder of Allan Chappelow, the prize-winning author of several books on George Bernard Shaw. Professor Slapper said: “The balance between on the one hand giving the defendant a fair trial by allowing him to know all the details of the case against him, and, on the other hand, keeping secret things that would endanger public safety or national security if generally known, is a difficult one.”

But it was the “great merit” of the system, he added, that it is the judge — not prosecution or defence — who will decide. “And judges, happily, as various home secretaries have discovered in recent history, are not factotums of the government or the security services."

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