The report argues the CPS "ignores" the true costs of employing barristers
The Crown Prosecution Service (CPS) has been accused of "Alice in Wonderland accounting" for claiming it saved millions by using its own lawyers.
The CPS had said using its in-house lawyers for prosecutions, rather than hiring outside barristers, saved taxpayers £11.5m last financial year.
But a study commissioned by the Bar Council said the CPS figures were flawed and unreliable.
The CPS has defended its policy and disputes the findings in the report.
Crown Court hearings in England and Wales used to be prosecuted by members of the independent Bar - self-employed barristers specially-trained to advocate in cases.
But over the past five years an increasing number of prosecutions have been conducted by lawyers working at the CPS.
And according to the CPS in the last financial year this saved taxpayers £11.5m, up from £8.4m the year before.
But a critical report by the consultancy firm Europe Economics says the CPS figures not only do not conform to government accounting standards, they are "so profoundly flawed" they "should not be relied on".
Bar Council chairman Desmond Browne QC told BBC Radio 4's Today programme the CPS had ignored the fixed overheads of employing in-house barristers - particularly accommodation and administration costs.
He said: "This leads to a skewed comparison - what they call loading the dice - in that the cost comparison will always favour the in-house advocate over the self-employed."
Keir Starmer QC, director of public prosecutions and head of the CPS, disputed the claims, saying its figures had been verified by the independent Inspectorate of Prosecution which found its methods "robust".
"I stand by our figures and our analysis," he said. "Most of the Bar Council's conclusions are unfortunately based on an incorrect assumption - namely that CPS accommodation and other costs would increase as a consequence of employing more CPS advocates.
"This has not and will not be the case. So their conclusions are simply wrong.
"In the last four years, the CPS spent over £500m on counsel fees and less than £40m prosecuting cases in house. It is evident where we should be looking to secure better value for money."
He added using in-house lawyers enhanced their skills, allowed the very best to be recruited and offered value for money.