PricewaterhouseCoopers has objected to ringfencing proposals from the Parliamentary Commission on Banking Standards.
Steve Davies, UK retail banking leader at PwC, said he was "unconvinced" that ringfencing would have any impact in stopping bank failures or a run on a bank.
Last week, chairman of the commission Andrew Tyrie MP said that he thought ringfencing between investment banks and retail banks should be carried out but that it could go further.
The commission wants a reserve power for full separation which would provide incentives for the banks to comply with the rules of the ring-fence.
Mr Tyrie said: "The proposals, as they stand, fall well short of what is required. Over time, the ring fence will be tested and challenged by the banks. Politicians too could succumb to lobbying from banks and others, adding to pressure to put holes in the ring fence.
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"For the ring fence to succeed, banks need to be discouraged from gaming the rules. All history tells us they will do this unless incentivised not to."
Mr Davies said: "The Commission's first report is an interesting contribution to the debate about how to implement ringfencing, however in highlighting issues such as derivative ringfencing, the Commission doesn't appear to be addressing the banks' and their customers' urgent need for clarity, but is instead raising further questions around the White Paper.
"We remain unconvinced that, other than assisting in an orderly resolution of a failing bank, the ringfencing approach would really have any impact in preventing a bank failure, preventing a run on a bank, or preventing egregious practices that would place the bank's future at risk.
"Implementing these complex proposals will take time, distract management and cost money."

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