Ratings agency Fitch cut the credit score of bailed-out lenders Lloyds and Royal Bank of Scotland, saying the government had become less likely to give them further financial support.
Fitch's downgrade of Lloyds and RBS followed a similar move last week from rival Moody's, which also cited a reduced likelihood of additional state assistance for the banking sector.
"Support dynamics are changing in the UK," Fitch said.
"The banking system is not only large relative to the UK economy, but there is also more advanced political will to reduce the implicit support for the country's banks."
Rating agencies had been widely expected to downgrade British banks amid signs the government's commitment to supporting them has waned.
The Independent Commission on Banking's recommendation in September that banks ring-fence their retail units from riskier investment banking operations and hold more capital overall, has also been seen as negative for their credit rating.
Lloyds and RBS are 41 per cent and 83 per cent state-owned, respectively, after receiving billions of pounds aid during the 2008 financial crisis.
Fitch also placed rival bank Barclays on "rating watch negative," signalling it too might be downgraded, citing exposure to volatile, market-sensitive business activities.