Criminals could be walking free because of a change in the way that suspects are charged at police stations, it has been claimed.
Police chiefs say that they are alarmed at how the Crown Prosecution Service is performing after it took over the task of charging people from officers. The police have now launched an internal investigation.
Amid concern that prosecutors are dropping cases that could have ended in a successful prosecution, the Association of Chief Police Officers (ACPO) has asked each of the 43 forces in England and Wales for its experiences of the new system.
Chief Supt Peter Hall, who is writing the ACPO report, said: "Not everyone is happy with the new charging scheme. There are worries about prosecutors' skill and competence. Charging arrangements need to improve."
Unhappiness at the performance of prosecutors extends to the grassroots. "It's a shambles," said Alan Gordon, the vice-chairman of the Police Federation. "The CPS charges people only when it knows it can get an easy conviction."
The row comes days after the CPS was criticised for not prosecuting the model Kate Moss for allegedly taking cocaine.
Following an eight-month investigation, the CPS decided that although video evidence provided "an absolutely clear indication that Ms Moss was using controlled drugs and providing them to others", the "precise nature" of the drug could not be established and therefore there was "insufficient evidence" to prosecute.
Under the previous system, police officers would charge offenders and then pass the case to the CPS, which decided whether to take the case to court.
Now, however, CPS lawyers work in police stations and they, rather than officers, decide whether to charge a suspect and what that charge should be. They can also tell police what evidence is needed to make a conviction more likely.
The ?31 million-a-year "statutory charging" scheme was introduced two years ago. It was intended to improve conviction rates and reduce the number of trials being discontinued.
The CPS insists that statutory charging has been a success, with the conviction rate rising from 78 per cent in 2003 to 83 per cent this year.
However, there is concern that the CPS is "cherry-picking" the cases most likely to succeed, or reducing the severity of a charge in order to ensure a conviction.
"It's causing inordinate delays in custody suites, resulting in more people being let out on bail while the police look for more evidence," Mr Gordon said. "The CPS will charge someone with common assault rather than grievous bodily harm because they will easily secure a conviction rather than taking the hard option and getting a conviction for what the offence actually was."
Mr Hall said: "There are, and have always been, differences of opinion between police and CPS prosecutors in the level of evidence you need to prosecute successfully.
"Police officers - and others in the criminal justice system - believe that the threshold has been raised. Sometimes we are satisfied with cases and sometimes not."
Officers are also concerned about CPS Direct, an out-of-hours service established to allow CPS lawyers, who work at police stations only during normal office hours, to instruct police from home.
According to Mr Hall, computer problems that make it difficult to transfer vital information have been reported.
Several lawyers contacted joined the criticism of the statutory charging scheme. "I think the CPS is cherry-picking cases," said a London defence solicitor who asked not to be named.
"Sometimes I think, 'Why are you not charging?' and I can't believe it. There are matters - such as actual bodily harm or class-A drug offences - which would have gone to court but don't. I have seen clients laugh and walk away."
A criminal barrister who also asked to remain anonymous said: "In theory I like the idea of the CPS being more involved at an earlier stage but the way it has been done is terrible.
"Police are telling me that the CPS always errs on the side of caution: it has targets for a higher conviction rate."
A CPS spokesman said: "CPS prosecutors do not cherry-pick cases nor are lawyers more cautious than police. There are no targets for conviction rates.
All decisions on whether a defendant is prosecuted or not are based on whether there is sufficient evidence for a realistic prospect of conviction and whether it is in the public interest to do so."
However, she accepted that there had been computer problems and that the new charging scheme had been a significant change for both police and CPS.