In January 2006 Leon Adams, my neighbour's son, had an ornate floral tattoo put on his neck. At its centre were the initials “H.N.I.C.” When his mother asked him what they meant, he found it funny that she didn't know. He told her that they stood for “Head Nigger In Charge”.

Anita is the first to admit that Leon was no saint. He'd been in trouble with the police and the tattoo sent out a message that worried her. She was right. Two weeks later her son was shot in the head and killed. The person suspected of the shooting was later shot dead in Trinidad, but others are believed to be involved and the investigation is continuing.

Until last week the police and the Crown Prosecution Service had a powerful weapon to bring dangerous criminals like those responsible for Leon's death to justice. Subject to judicial approval, they could offer frightened witnesses complete anonymity. On Wednesday the law lords effectively took that weapon away. The ruling is having a dramatic effect on murder investigations, and some may not now come to trial. It could also lead to a rash of appeals. No wonder Jack Straw, the Justice Secretary, has hinted strongly that emergency legislation will be introduced. But any legislation drawn up in haste could prove disastrous.

Anonymity kicks in when witnesses first give a statement to police under a pseudonym. They give evidence in court from behind screens, visible only to the judge and jury. Their voices are distorted so that it is impossible to tell their race, age or gender. Crucially, the witness cannot be seen, heard or identified by the accused, or the accused's associates in the public gallery.

Amid the horrendous spate of gun and knife murders, anonymity was seen as a vital tool in bringing those responsible for the most brutal crimes to justice. But it worried many lawyers. Although the prosecution was under a duty to disclose any evidence about the witness's credibility, this was inevitably constrained, because it couldn't disclose material that would identify the person. That put severe limits on defence counsels' ability to cross-examine witnesses about their honesty. And there were other problems. Defence counsels could not ask other witnesses if the anonymous witness was at the murder scene. To do so would reveal the identity.

Nigel Rumfitt, QC, who defended one of those accused of the murder of Letitia Shakespeare and Charlene Ellis, says: “Once the jury see a witness screened off and with their voice distorted, you worry that, despite warnings from the judge and prosecutor not to do so, they will simply assume that the defendant is a dangerous criminal capable of serious violence.”

There were also real concerns that fellow gang members wishing to curry favour with the police would volunteer as a witness and settle scores under anonymity. It's cleaner and less dangerous than a messy shooting after all.

A few weeks ago I went out late on a Saturday evening with officers from the Metropolitan Police's Operation Trident, which investigates shootings in the black community. They were trying to glean information and find witnesses to the fatal shooting of 17-year-old Sharmaarke Hassan in Camden Town, northwest London. The reluctance of people to talk was palpable and worrying.

But is intimidation any worse than in the past? The law lords point out it has been around since the time of Cicero, while modern domestic cases are all very recent. Lord Diplock considered anonymity during the extreme problems of witness intimidation in Northern Ireland in the early 1970s and rejected it as a step too far. The Americans have managed to dismantle large sections of the New York mafia without witness anonymity.

The law lords ruling in R v Davis traces the long-established principle of our common law that a defendant has the right to know and see his accusers. Without it, cross-examination was likened to “taking blind shots at a hidden target”. The European Convention on Human Rights (ECHR) also guarantees a defendant the right to examine or have examined witnesses against him. Strasbourg case law also establishes that no conviction should be based solely or to a decisive extent upon anonymous testimony.

The Government's problem is considerable. It must draft a statute that delicately balances the need to protect witnesses in the prosecution of dangerous criminals capable of violent intimidation, against the defendant's rights established in common law, the ECHR and European case law.

If it fails, it faces two equally dire consequences. Either ministers will find themselves on the receiving end of another human rights reverse by the House of Lords. Or they will find themselves overseeing a system in which the killers of Leon Adams, Sharmaarke Hassan and many others can never be brought to justice.

Clive Coleman is a barrister and the presenter of Law In Action on Radio 4.

Times Recommends

• Resist urge to throw voters red meat
• Slay sacred cows and be candid
• We cannot let these 100 deaths deflect us