George Osborne's staff claim the chancellor has "laid down the law" to bankers, saying that fines for the Libor rate-rigging scandal must come out of their bonuses.
State-backed Royal Bank of Scotland (RBS) is braced for a huge fine from American regulators, as it becomes the third bank to settle over its traders’ role in the scandal.
The Government still owns four fifths of the bank, saved from collapse by a £45bn bailout four years ago, in the wake of the ill-advised acquisition of Dutch bank ABN Amro.
Senior RBS figures were warned that leaving taxpayers to cover the US penalty for the bank's role in fixing the lending rate, which governs the price of more than $500 trillion-worth of loans and transactions around the world, would be "totally unacceptable".
RBS is rumoured to be preparing to hold back on some perks in preparation for the fines, and is thought to be close to reaching a deal with regulators in Britain, America, Japan and Singapore. It faces a total fine of £350 million.
A senior Treasury source said: "Fixing the Libor market is a symbol of all that went wrong with the banking system over the past 10 years. We are now putting those things right.
"Ahead of any other country, we legislated to make abuse of the system a criminal offence and are stripping the banks of the power to administer it themselves.
"The authorities are rightly pursuing those individuals who abused the system; and the regulators are rightly fining the banks they worked for.
"We all know there are Libor investigations ongoing into RBS in both the UK and the US. Any UK fine will already go to the public, and the Chancellor has made it clear that on this occasion the bill for any US fine should be paid for by the bankers, and not the taxpayer."
Last year Barclays was fined a total of £290 million, of which £58.5 million was in the UK, while UBS was hit by a global fine of £940 million, including a £160 million UK penalty.