It took less than a week. Last Wednesday, the law lords gave Iain Davis leave to appeal his murder conviction on the ground that it had been obtained with anonymous evidence. Yesterday the first continuing murder trial to be affected by their lordships' ruling fell apart.

There will be more trials like it, and prisoners who may be guilty may walk free. There are strategies that can and must be adopted for preventing this, by protecting witnesses without disguising their identities. But in the meantime the confusion that has followed the Davis ruling is a price worth paying for the preservation of a basic principle of British justice.

In less than two decades, with no statutory basis, the use of witness anonymity in British trials for serious and violent crime has grown from negligible to commonplace. One QC involved in the Davis defence estimates that applications for anonymity are made in three quarters of all his cases. The reasons are clear. Faced with worsening gun and knife crime and pitifully low conviction rates in some of the country's most blighted urban areas, police have found offering complete anonymity by far the most effective way of encouraging fearful witnesses to come forward. The trend began in the early 1990s, when a judge first allowed prosecutors to withhold a witness's identity as well as disguise it with “special measures” (such as the use of screens and voice distortion) for which there was already provision in law. One senior barrister complained yesterday that the use of anonymity in murder trials has since become routine.

The results, at least in terms of charges brought, have been impressive. Detection rates in gang-related murder cases have more than doubled in parts of London since police started offering witnesses anonymity as part of Operation Trident in 1998. Since last week senior police have been unanimous in their warnings that the law lords' ruling risks undoing years of effort building trust between detectives and troubled communities - and there is little doubt that yesterday's abandonment of the £6 million Charles Butler murder trial will be followed by other aborted trials and a wave of appeals.

Small wonder that Jack Straw, the Justice Minister, wants to change the law. Emergency legislation to give explicit statutory backing for anonymous evidence has been, he says, his “major preoccupation” since last week. His aides say it may even be ready for the Queen's Speech in December. But he and they should move cautiously.

The right of the accused to confront and cross-examine an accuser is not just an ancient principle. It is as vital for fairness as it has ever been. Without knowing who a hostile witness is, no defence lawyer can properly assess his or her credibility for a jury. Anonymity is therefore not just a defence against intimidation. It can actually serve as an inducement to perjury, especially in trials for gang-related crime in which witnesses may use it to settle scores. Furthermore, it can prejudice a jury against a defendant as evidence of fear that he or she inspires, whether or not that fear is proven.

There was no provision for witness anonymity throughout the Troubles in Northern Ireland. There is none in the US, or at the International Criminal Tribunals set up for the former Yugoslavia and the Rwandan genocide. In each case, intimidated witnesses are identified - but protected. It is not yet clear that Britain needs a witness protection programme as costly and comprehensive as America's. But it is clear that intimidated witnesses must be offered physical protections as robust as necessary to persuade them to testify in their own names. Anonymity erodes the guarantee of a fair trial, and that will always be too high a price to pay for a conviction.