But if its performance has improved in recent years, today’s figures showing big discrepancies in the success rates in prosecuting offenders will revive all the old criticisms. As many as 41,000 cases are thrown out at the door of the court because of insufficient evidence.

The statistics will resurrect debate about the CPS’s performance at a time when it is pushing for a much bigger role. Unlike district attorneys in the US, crown prosecutors are not political and do not have to watch their results to secure their jobs. But the CPS inspectorate is planning its own grading system, from “excellent” through to “poor” — the nearest thing yet to league tables for the performance of prosecutors. And judged by the standards of a few years ago, the scores can only look good.

The CPS started life with one hand tied behind its back: when set up 1986 as the first independent prosecution service in England and Wales, it had inadequate resources and too few staff. It took years to dispel its image as the whipping boy of the criminal justice system.

Now it is doing better after securing more money, recruiting better-quality staff and, crucially, embarking on a new strategy to put itself “centre stage”.

Its recovery began with Sir David Calvert-Smith, DPP from 1998 to 2003, who won Treasury funding and removed much red tape. Since then the current DPP, Ken Macdonald, QC, has overseen an expansion of the CPS to take on charging offenders, away from police.

He defends the average conviction rate of 82 per cent, arguing that no one would want to live in a country where it was 100 per cent. But he accepts the rates are still too low for crimes such as rape and other sexual offences. More than one third end in failure; in some areas the figure is almost half.

Reforms are under way to tackle those cases and to boost the role of prosecutors, giving them the powers that continental prosecutors have had for years, to interview their own witnesses.

He also has far-reaching ambitions for the service, including a “quasi-judicial”role, diverting offenders from court and imposing its own penalties.

The CPS came about after a series of high-profile miscarriages of justice. The consensus was that the investigation of crime had to be split from the prosecution and that it was inappropriate for the police to do both. Now the view is that the split is too great, and there should be measures to encourage closer working.

But the charging reforms are a first step in what Mr Macdonald calls a “basic building block in an entirely new architecture. “In essence,” he said last week, “prosecutors become the gatekeepers in the system. No case goes ahead unless it gets through us first.”

It is true that fewer cases are being discontinued and more guilty pleas being entered. There is a 15 per cent rise in convictions, 30 per cent rise in guilty pleas and 69 per cent drop in discontinuances. But victims and witnesses remember the statistics of failure.

The DPP knows this. He wants power to interview witnesses before trial — common practice in America, Canada, Australia, Scotland and Northern Ireland, so cases are less likely to collapse because a witness fails to come up to scratch.

And he favours prosecutors imposing their own penalties, including community sentences, if an offender is willing to plead guilty and avoid going to court.

That could remove thousands of cases from the courts and cut delays, he says. It could also do much to turn round the current disparate performance of the service itself.

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