Rishi Sunak has been accused of wasting £11billion of taxpayer cash by paying too much interest servicing the Government's debt.
The Chancellor was accused of failing to insure against interest rate rises on £900bn of government reserves.
The losses were highlighted by think tank the National Institute of Economic and Social Research (NIESR).
They are said to exceed the amount the Tories accused Gordon Brown of losing when he sold some UK gold at rock-bottom prices.
NIESR director Professor Jagjit Chadha told the Financial Times Mr Sunak's actions had left the country with "an enormous bill and heavy continuing exposure to interest rate risk".
Labour said the losses were "astronomical" and accused the Government of "playing fast and loose" with the public finances.
But Economic Secretary to the Treasury John Glen said the reports were “inaccurate”.
He tweeted: “The proposals are complicated and involve forcing banks to swap reserves for longer-dated securities, but the £11 billion figure itself is based on almost impossible scenarios and implementing the proposals would have a significant impact on market prices and credibility.”
A Downing Street spokesman added: “It’s simply not true.
“The measures that are proposed would have come with huge economic risks, and would have undermined the Bank of England’s independence.
“Specifically, forcing commercial banks to swap reserves for gilts undermines the independence of the Bank of England and would be an act of financial repression.
“And the £11bn figure itself is based on an implausible assumption.”
He said what’s not true is that “the figure that has been given could have been saved in the way they have set out.
“For example, getting to that figure requires perfect foresight of market prices, it essentially says two years ago you’d have known exactly where the bank’s rate and bond yields would be now.”
Shadow Chancellor Rachel Reeves said: “These are jaw-dropping sums.
“£11 billion lost to fraudsters and criminals and now this. Labour will get a grip of this Conservative government’s wasteful spending and treat taxpayers’ money with the respect it deserves.”
According to the FT report, the Bank of England created £895bn through quantitative easing - pumping money into the economy,
Most of this was used to buy government bonds from pension funds and other investors.
When those investors put the proceeds in commercial bank deposits at the BoE, the Bank had to pay interest at its official interest rate.
Last year, when the official rate was still 0.1%, the NIESR urged the Government to insure the risk interest rates would rise higher.
Since then, the Bank of England base rate has risen repeatedly and now stands at 1%.
Prof Chadha said the NIESR had now calculated Mr Sunak's failure to heed their advice - despite having regularly warned about the risks of higher inflation and interest rates on the costs of servicing the Government's debt - had cost taxpayers £11 billion.
"It would have been much better to have reduced the scale of short-term liabilities earlier, as we argued for some time, and to exploit the benefits of longer-term debt issuance," he told the FT.
Shadow treasury minister Tulip Siddiq said: "These are astronomical sums for the Chancellor to lose, and leaves working people picking up the cheque for his severe wastefulness while he hikes their taxes in the middle of a cost-of-living crisis.
"This Government has played fast and loose with taxpayers' money. Britain deserves a government that respects public money and delivers for people across the country."
