Paul Volcker, the former Federal Reserve chairman, expressed more doubts over the White House’s plan for financial regulatory reform on Thursday and backed new taxes on banks.

Mr Volcker, chairman of the Fed between 1979 and 1987, said he was concerned that the financial sector had grown out of proportion to the US economy. The White House adviser said he was “very interested” by ideas for a tax on transactions between banks, which was floated on Thursday by Peer Steinbrück, German finance minister, and recently by Lord Turner, head of the UK Financial Services Authority.