There are clearly problems with the way that British courts are dealing with supergrasses – criminals who give evidence against their former accomplices. The system of doing deals so that witnesses who shop their fellow villains get shorter sentences was formalised under the 2005 Serious Organised Crime and Police Act. Since then, the authorities have signed deals with 158 supergrasses. An investigation by the Bureau of Investigative Journalism has analysed 49 of these and found that in nearly half, the jail terms were cut by more than two thirds. Among these supergrasses were murderers, gangsters and drug dealers.

The argument for the system is that the use of informers is an essential tool in the fight against organised crime. Lesser punishments for small-fry criminals are a necessary sacrifice to secure the conviction of more serious offenders and ringleaders. What has raised concern are the examples the Bureau has uncovered. In one case, life sentences for two murderers were reduced to only three years despite their testimony being branded by the judge as "infected with lies". In another, a murderer's sentence was cut from 18 to eight years only for a jury to refuse to believe his testimony and acquit the other defendants.

But it is important not to throw the baby out with the bathwater. The aim of the justice system is not just to punish. It must also reduce crime and make the public safer.

A supergrass system gives offenders an incentive to manipulate the system. But that can be a price worth paying if it secures the conviction of bigger criminals. The murderer of the Everton schoolboy Rhys Jones was found because a deal was done with a youth who had helped hide the murder weapon.

There will, of course, sometimes be unforeseen turns of events in such cases. But the current system offers an increased transparency, which enhances the credibility of the testimony. It should be applied with more care rather than being discredited by a few bad examples.