Creditworthiness ratings of 12 British financial firms, including Lloyds TBS, Nationwide, RBS and Santander-UK, were downgraded, Moody's said Friday.
The downgrades were prompted by "Moody's reassessment of the support environment in [Britain], which has resulted in the removal of systemic support for seven smaller institutions and the reduction of systemic support ... for five larger, more systemically important financial institutions," Moody's Investor Services said in a statement.
Moody's said it believes the British government was less likely to support some firms if they experienced trouble, the BBC reported.
The agency said the downgrades didn't "reflect a deterioration in the financial strength of the banking system."
Chancellor of the Exchequer George Osborne said one reason for the downgrades was because the government was viewed as "trying to deal with the too-big-to-fail problem."
"One of the reasons they're doing this is because they think the British government is actually moving in the direction of trying to get away from guaranteeing all the largest banks in Britain," Osborne told the BBC. "People ask me how are you going to avoid Britain and the British taxpayer bailing out banks in the future? This government is taking steps to do that."
He expressed confidence that British banks were well-capitalized.
"They are not experiencing the kinds of problems that some of the banks in the eurozone are experiencing at the moment," Osborne said.