Rishi Sunak has been accused of failing to act soon enough to save £11bn of taxpayers' money that has been used to pay interest on government debt.
The National Institute of Economic and Social Research (NIESR) said the losses stemmed from the chancellor's failure to insure against interest rate rises.
It meant higher than necessary payments on £900bn of reserves created through the quantitative easing (QE) programme.
The Treasury said it was the Bank of England's role to decide QE measures.
NIESR's Prof Jagjit Chadha, told the Financial Times that Mr Sunak's actions had left the country with "an enormous bill and heavy continuing exposure to interest rate risk".
