UK Finance's letter to the Chancellor, John Healey, puts the cost of raising the bank surcharge to 8% at about £6bn of lost UK output over five years, GB News reports.
According to GB News, the lobby group's letter says the rise would mean "around £6bn of UK output forgone over the five-year Budget forecast period, rising to nearly £20bn over 10 years".
The same letter had already been reported as estimating that annual real GDP would be £1.2bn lower by 2029-30. The new figures are cumulative totals over the two periods.
Experts quoted by GB News say the roughly £2bn a year the rise is reported to raise is wrong, because of that cost to the economy. The Guardian first reported the £2bn figure. The Treasury has not confirmed any change to bank taxes ahead of the Budget on 28 October.
UK Finance lobbies on behalf of the banking industry. Its figures are not OBR forecasts and have not been independently checked.