Moody's has said it may cut its ratings of 19 UK banks to reflect the lower chance of a future government bail-out.
The rating agency said the change was likely to hit smaller lenders, including building societies, first.
The big four banks - typically seen as the most systemically important because of their size - will be assessed only in the second half of the year.
The banks currently enjoy ratings up to five notches higher than they would otherwise be due to "systemic support".
"This reassessment could trigger negative outlooks, reviews for possible downgrade, or downgrades for some ratings, and will be taking into consideration Moody's expectations on how the relevant banks' standalone credit strength will develop," said the rating agency.
Any downgrade is likely to increase a bank's cost of borrowing, because many investors are restricted from lending to borrowers with lower ratings.
Before the crisis, the maximum such support assumed for the biggest and most systemically-important banks was two notches, implying that some could see their credit ratings cut by three notches.
Currently, even some small lenders, such as building societies, enjoy a high level of support, although in normal times they would be considered the least likely to get bailed out.
In the worst case, downgrades could leave some of these smaller financial institutions with high-risk "junk" credit ratings.
The agency pointed out that the reassessment did not reflect any deterioration in the banks' performance or the financial strength of the UK government.
Instead it was due to the changing regulatory environment, which Moody's believes will reduce the chances of UK banks enjoying financial rescues from the government in the future.
"We're taking stock," said Elisabeth Rudman, the lead analyst for UK banks at Moody's, who explained that the agency wanted to set the scene for possible rating actions so that they would not catch markets by surprise.
The Vickers report into banking, due out next week, is likely to influence any decision.
It is expected to recommend a split between the big banks' investment banking and commercial banking businesses.
She also said that the EU's planned new framework for managing banking crises could affect any rating decision, as the new European rules could mean bigger losses for a failed bank's creditors.