Chancellor John Healey is planning to increase taxes on the bank sector, senior banking executives have told the BBC.
Healey has described banks as part of "the crown jewels" of the UK economy, but leaders in the sector have said it will be "on the menu" at the Budget in two weeks' time.
The biggest four banks collectively made about £15bn profit in the first six months of this year, and trade unions have called for them to face additional taxes.
The trade body for the banking sector, UK Finance, warned "further increases in bank taxes would weaken competitiveness, discourage investment and job creation, and work against the government's ambition to deliver growth in every postcode".
Shares in banks including Lloyds and NatWest fell on Friday on expectations that the sector would be hit by tax hikes. The firms were among the biggest fallers on the UK stock market, with both dropping more than 2%.