Royal Bank of Scotland and Lloyds are among fourteen UK banks and building societies whose credit ratings face being cut because of government support withdrawal. The downgrade would increase borrowing costs for such lenders.
Santander UK and Nationwide Building Society may also suffer a downgrade following the debt ratings review of UK banks carried out by Moody's Investors Service. Currently Lloyds, RBS and Santander's debt is Aa3 rated; Nationwide's Aa3/P-1.
A senior credit officer for Moody's said: "The reassessment is not driven by either a deterioration in the financial strength of the banking system or that of the government. It has been initiated in response to ongoing guidance from the UK authorities - the Bank of England, the Financial Services Authority and the Treasury."
The surprise announcement comes after a Bank of England decision not to roll over the Special Liquidity Scheme emergency fund when it expires in January of next year.
Barclays and HSBC will not feature in the Moody's review.