This echoes comments made by the Bank of England, which at the end of November called on banks to do a full valuation of their loan books to ensure they have enough capital.
"Fitch has some concerns over the very low risk weightings assigned to loans backed by real estate (residential and commercial mortgages), which are being reviewed by the UK regulator," the ratings agency said yesterday.
Fitch said it was worried about interest-only mortgages, which make up 46% of all home loans, and the impact of an unexpected spike in unemployment.
But it said the main vulnerability lay in banks' books of commercial property loans which were amassed ahead of the financial crisis.
Fitch noted that a large portion of these were the subject of renegotiation, refinancing as sale and that property prices remained under pressure.
It drew attention to the Irish property books of part-nationalised Lloyds Banking Group, owner of Bank of Scotland, and especially that of Edinburgh-based RBS, which owns Ulster Bank.
As of June 30, Lloyds had put aside £6.2 billion to cover write-offs on its £10bn book of Irish commercial property loans.
RBS has £44.8bn of gross loans against which £10.1bn provisions are held.
The major UK banks are "generally soundly capitalised", Fitch said. But it said they would likely have to set aside more, either by way of additional capital buffers or higher risk weightings, for certain classes of loans.
Earlier this month, Bank of England Governor Sir Mervyn King said the US had been faster and better at recapitalising its banking system than the UK.
Meanwhile, it was reported that Lloyds is to repay between €8bn (£6.8bn) and €10bn of the long-term cash it borrowed last year from the European Central Bank.
Such a move would be a signal that it believes it has a strong balance sheet and liquidity position so does not need the cash.