The CPS Inspectorate's report said the CPS was slow to contact victims in almost half of the cases it examined and sent letters that "lacked empathy".
In a majority of cases there was also no evidence that victims' views were taken into account on key decisions.
The CPS said its own data showed a "markedly different" picture to the "small sample" used in the report.
Her Majesty's Crown Prosecution Service Inspectorate's communicating with victims report looked at the speed and quality of the CPS's communication with victims of crime.
As part of its investigation, inspectors visited six of the 13 CPS areas in England and Wales, during a three-week period. They examined 162 case and complaint files and interviewed CPS staff, police and crime commissioners and partner organisations such as Citizens Advice.
The Inspectorate said the CPS should be more consistent in the quality of their letters and how quickly they send them to victims, and added the CPS and police should agree on their roles in communicating case outcomes to victims to avoid sending confusing or duplicated information.
They also stated the CPS should "implement an effective process" for notifying victim liaison units when charges are altered in a case, so that staff can inform victims.
Inspectors did say the CPS was facing increased pressure due to staff cuts, and the report also highlighted that the organisation had taken steps to improve service, including drafting new guidance about speaking to witnesses at court to improve the quality of service.
But the CPS criticised the small sample used in the report and said their own figures showed they had improved service.
Chief crown prosecutor Martin Goldman said the CPS is "committed to delivering a high quality service to victims and witnesses and supporting them throughout their case".
He said: "Last year a survey showed that two-thirds of victims and three-quarters of witnesses were satisfied or very satisfied with the service they received from the CPS.
"80% of letters to victims due to reach them within one day meet the target, and this increases to 89% of letters due to reach victims within five days."
Victims of crime are "let down" by poor communication from the Crown Prosecution Service, a watchdog claims.