When two British Army corporals ventured into West Belfast in 1988, were dragged from their car and beaten to death, the prosecution called several television journalists to give evidence.

The trial judge and law lord Sir Brian (now Lord) Hutton agreed that they should not be identified by name and that when giving evidence they could be screened, so that their faces would be seen only by the judge and lawyers.

That case is thought to be the first in which judges explicitly recognised that in some exceptional situations there could be a departure from the principle of open justice.

The step was a small one: the witnesses’ evidence did not implicate the defendants in the crime; nor was the witnesses’ credibility in question.

But in the past decade, that small step became a stride. One criminal QC, Malcolm Swift, estimated this week that three in five murder cases now involve anonymity for a witness claiming to be at risk of intimidation.

The practice has grown in parallel with a gangland culture in which potential witnesses fear speaking out. It was developed for the best of intentions – to secure justice – but it cuts across a fundamental common law: the right of an accused to know who his or her accuser is.

This conflict between the need to secure a conviction and a defendant’s right to a fair trial came to a head in 2005 when four killers were jailed for life for the shooting of two teenage girls, Charlene Ellis and Letisha Shakespeare.

Detectives hailed the six-month trial as a watershed in the fight against gang leaders. But the use of witness-protection measures brought rumblings of protest. One solicitor condemned the trial as one of the most unfair in history.

Last week the law lords restated the law. As they quashed a double-murder conviction, secured after witnesses fearing for their lives were granted anonymity, they said that it was a long-established principle of common law that an accused can confront his or her accusers.

If such anonymity was to be granted, then there needed to be legislation, the law lords added.

Their ruling does not affect special measures that took effect in 2002 under the Youth Justice and Criminal Justice Act 1999 for vulnerable witnesses (children, victims alleging rape) who can give evidence via video-links or from behind screens.

What it does affect, though, is the growing number of applications by witnesses who seek anonymity on the ground that otherwise they could not come forward.

As the law lords said, it is a slippery slope. Citing Mr Justice Richardson in the Court of Appeal of New Zealand in 1986, they said: “Today the claim is that the name of the witness need not be given: tomorrow, and by the same logic, it will be that the risk of physical identification of the witness must be eliminated in the interests of justice in the detection and prosecution of crime, either by allowing the witness to testify with anonymnity, for example from behind a screen, in which case his demeanour could not be observed, or by removing the accused from court, or both.”

Mr Swift, counsel for the defendant in the House of Lords case, said: “After those prophetic words were uttered, our courts began to descend that slippery slope. The need to convict those accused of crime was the driving force. Witness anonymity was one of the means employed.”

The law lords did not change the law; they reasserted it. If ministers want it otherwise, they will need to legislate, the law lords said. But if ministers legislate, they must ensure that judges can still decide when the balance tips in favour of discarding a basic principle of justice.

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