The performance of the Crown Prosecution Service (CPS) South Wales is poor and has declined since 2007, an inspection has concluded.
The report said cases needed to be prepared to a better standard in both crown and magistrates courts.
However, inspectors said they were encouraged by the new management's "determination to get to grips with the challenges that have been highlighted".
CPS South Wales said it had "moved on" since the inspection.
The report said relationships with some key criminal justice partners needed to improve but action was being taken to address the issue.
Crown court casework and preparation of magistrates court cases for trials were identified as areas that needed improving.
There were also "significant budgetary challenges", and CPS South Wales needed to "review its structures and aspects of staff deployment including the way it deploys its crown advocates".
Aspects of the service rated as poor included pre-charge advice and decisions; decision-making, preparation and progression in magistrates court cases; custody time limits and disclosure of unused material.
A significant proportion of cases were prepared late, the report found.
Some improvements were noted, such as the service provided in cases involving rape and violence against women, and the quality of contact with victims generally.
The overall assessment of the service was downgraded from fair in 2007 to poor in 2010.
HM chief inspector Michael Fuller said: "Whilst it is disappointing to note that South Wales overall performance has declined since 2007, I am encouraged by the recent changes made and the new management team's determination to get to grips with the challenges that have been highlighted in our report.
"Despite the overall rating of poor, the area has had a measure of success in some aspects of work, such as improving the service provided in cases involving rape and violence against women and the quality of its contact with victims generally.
"The area must focus on improving both its crown court casework and preparation of magistrates courts cases for trial, and we have made a number of recommendations to address the quality, as well as the timeliness, of preparing cases for court.
"Budgetary control and efficiency remain a key challenge for the area into the next financial year and beyond. We have made recommendations to address this issue and which, if implemented, will lead to savings."
Interim chief crown prosecutor for south Wales, Jim Brisbane, said he recognised the importance of continually improving the service to the communities it served.
"We have moved on since the inspection and, although the report does not make easy reading in parts, we acknowledge the findings of the inspectors and accept the recommendations they make," he said.
"While the overall rating of poor is disappointing, particularly for our hard-working staff, I am heartened that the report acknowledges the changes that have already been put in place to improve the efficiency of CPS South Wales.
"I am also pleased that elements of our work, for example in the fields of rape, violence against women and community confidence, are rated as good."
Since the inspection, CPS South Wales has reviewed its management and operational structures and brought in a new "performance monitoring system".
It has also started collaborative work, particularly with police, to "increase efficiency" and improve the way organisations in the criminal justice system work together.
Improvement plans have been worked out to address the main areas of concern highlighted.