Too many Crown Prosecution Service barristers lack "presence, self-confidence and flair" in Crown Court trials, a report has said.
The CPS Inspectorate found that CPS barristers in England and Wales were in danger of "losing" the jury because of how they presented cases.
Standards of CPS advocacy had taken a "step backwards" over the past three years, the report said.
The CPS said it disputed this and said conviction rates had been maintained.
Chief inspector Michael Fuller said the review took place against "a significant background of change" for the CPS with budget cuts and a reduction in the number of CPS advocates.
But he said a reliance on outside barristers to present cases posed a "real threat to in-house prosecutors' courtroom skills".
"This is compounded by the lack of exposure to Crown Court advocacy and scant opportunity to progress to crown advocacy," he said.
"This means with things as they are, there will be an inevitable drop in prosecution standards in the courts.
"The CPS needs to evaluate its advocacy provision."
A CPS spokesperson said: "We dispute criticism of the quality of CPS advocates and are disappointed that this finding has been made when the Inspectorate undertook no observations of advocates in action.
"The report's findings are based on our own assessment of advocates, which is specifically targeted at those that we believe need it most.
"It is, therefore, an inaccurate picture of the overall quality of our advocates."