Police have turned to “supergrasses” once again in the fight against organised crime, reviving a practice that was discredited in the 1980s after many convictions were quashed on appeal.

Informers who have provided crucial evidence to the Metropolitan Police “supergrass unit” have received significant reductions in their sentences.

One supergrass had his sentence cut from 60 years to only four, The Times has learnt, after giving evidence that helped to convict several high-profile criminals.

The unit, officially known as the Debrief Unit, is tackling those at the top end of the criminal scale who are involved in murders, drug and weapon smuggling and people-trafficking.

The unit claims that the scheme has had “a major impact on criminal networks in London”, being aimed at 30 leading gang bosses.

The term supergrass was coined in the 1970s to describe informers who helped to convict senior figures in gangland London.

The practice was used widely in Northern Ireland in the 1980s by the Royal Ulster Constabulary, which sought information from paramili-taries who had been arrested in exchange for immunity from prosecution.

More controversially, payments in cash were made in return for information. Dozens of IRA members were jailed after they were convicted on the strength of intelligence provided by informers. However, many convictions were overturned on appeal because the evidence was considered to be unsafe.

The Metropolitan Police, which is currently without a permanent commissioner after the resignation of Sir Ian Blair, accepts that it must do everything it can to make sure that the high-risk scheme is not open to the abuses of the past.

Senior officers, who declined to say how many informers have helped the police, say that the current system is subject to much closer scrutiny than in the past.

Evidence put forward by a supergrass must be considered by the Crown Prosecution Service (CPS) before a deal is agreed. The vetting process can take several months and potential informers are obliged to surrender any financial proceeds that they have accrued from their criminal activities.

They must also admit all past crimes. The CPS will then decide whether or not to prosecute. However, if convicted, they will not go to jail - for their own security - but instead serve their sentences in secure premises, albeit subject to restrictions similar to those that would be experienced in prison.

Supergrasses must agree to give evidence in person in trials against their former colleagues and do not have recourse to special measures designed to protect their identity in the courtroom. Many choose to join the Witness Protection Scheme.

Retired Detective Superintendent Barry Phillips, the former head of the Debrief Unit, admits that there were many failings during the 1980s. “We became sloppy. We cut corners, we didn’t corroborate evidence, we didn’t get supporting evidence. The defence teams were becoming more astute at defending cases.

“This brought the whole supergrass system into disrepute. Then, when the CPS was created during the latter half of the 1980s it said it would not touch any supergrasses.

“It had become discredited - the whole process of taking an individual and turning him into a Crown prosecution witness, who will stand and give evidence against criminal associates.”

Detective Inspector Tony Moore, from the unit, said: “We get some people just coming to us off the street saying they have been threatened by a gang they are involved with and they have information, but they have to incriminate themselves and what they say has to check out.

“We get referrals every week. It ranges from an offender who wants to give Queen’s Evidence in a simple trial to those offenders who are in prison serving serious sentences and have had plenty of time to reflect and decide to offer information that might solve more robberies or murders.”

Not everyone is accepted into the system, however.

Mr Moore said: “Because of costs and witness protection, if we get someone who wants to go into the system we will look at the potential for them to attack the system and settle old scores.

“They have to admit to all previous criminality, even if it is something they have already been found not guilty of in a court.”

Deals have been agreed with men and women who had committed serious crimes, including murder, Mr Moore said.

“We have to balance what we do with families that have had someone murdered. They want to see a person who killed their loved one given a big sentence but if they help us to convict others and get a reduction on their sentence it may mean that several other families can get closure.”

The future of the supergrass was cemented in 2005 with the introduction of the Serious and Organised Crime and Police Act (Socpa), providing a legal framework for using testimony from “assisting offenders”.

Mr Phillips still prefers the term supergrass. “It’s a word that lets you zoom straight in and understand what is going on. It’s an old-fashioned term for an old-fashioned policing method that is now coming back into vogue.”

Times Recommends

• What makes the perfect mum?
• To keep me stress-free, no more awareness days, please
• What do lottery winners buy?