Police forces are spending more time and effort building cases than needed, with four in five prosecution files containing unnecessary paperwork, auditors have found.
However more than half still failed to provide an adequate summary of the prosecution case, the National Audit Office (NAO) said.
The findings come in a review which showed police officers often did not know which documents to include or exclude following the "flawed" national roll-out in 2008 of a scheme designed to cut the amount of paperwork in straightforward cases.
The initiative could save police forces £10 million by reducing the time spent to compile each file correctly by an hour, without reducing the number of guilty pleas or leading to delays, the report said. However it has so far failed to reach its potential, with variations in compliance across the 43 forces in England and Wales.
The auditors, in partnership with Her Majesty's Inspectorates of Constabulary and the Crown Prosecution Service (CPS), said the project was rolled out nationally before its pilots were completed and evaluated. The review of 100 case files found 79% of police files contained a disproportionate amount of paperwork, and more than half of the files (53%) did not summarise key evidence to a high enough standard.
Most police files were signed off by a supervisor, despite not complying with the guidance. When interviewed for the review, some supervisors said they did not have the time to read all the files that they signed off. The report also found police officers did not generally know which documents to include and CPS staff reportedly requested more evidence than the guidance recommended.
Amyas Morse, head of the NAO, said: "The streamlined process initiative holds the promise of cutting police paperwork, thereby saving money and freeing officers up for other tasks, without reducing the effectiveness of courts.
"But its roll-out did not follow the principles of good project management, it is unclear whether savings have been made and local police areas have not all bought in equally to the need to implement the guidance."
A Home Office spokesman said: "We have made clear our determination to slash police bureaucracy and return officers to the front line, so we welcome this report which has identified significant potential savings for forces.
"We will work closely with the police and the Crown Prosecution Service to ensure they are able to maximise these savings."