Moody's has announced that it has downgraded 12 of the UK 's financial firms including Lloyds TSB, Royal Bank of Scotland (RBS) and Santander UK .
The ratings agency said it had taken the decision because it thinks that the UK Government is now less likely to help out firms in financial difficulty.
Moody's did stress, however, that the downgrades did not reflect a deterioration in the strength of the banking system in this country or that of the Government.
Speaking to the BBC this morning, Chancellor George Osborne said he was confident about the UK 's banks and building societies, and that they were not as exposed to the eurozone debt crisis as some other institutions.
Nine Portuguese banks have also had their credit ratings cut.
The credit agency said it had been reviewing UK financial institutions since May this year.
In a statement it said: " Moody's Investors Service has today downgraded the senior debt and deposit ratings of 12 UK financial institutions and confirmed the ratings of one institution."
"The downgrades have been caused by Moody's reassessment of the support environment in the UK which has resulted in the removal of systemic support for seven smaller institutions and the reduction of systemic support by one to three notches for five larger, more systemically important financial institutions.
"According to Moody's, announcements made, as well as actions already taken by UK authorities have significantly reduced the predictability of support over the medium to long-term."
Of the banks to have their ratings cut, RBS and Nationwide have been downgraded by two notches, while Lloyds TSB and Santander's ratings were cut by one notch.
The other institutions to have their ratings cut were The Co-operative Bank, Clydesdale Bank, Newcastle BS, Norwich and Peterborough BS, Nottingham BS, Principality BS, Skipton BS, West Bromwich BS and Yorkshire BS.
As well as downgrading the banks and building societies, Moody's said that RBS and Lloyds would have a 'high likelihood of support' should they fall into trouble.
Those thought to have a 'moderate or high likelihood of support' were The Co-operative Bank, Clydesdale Bank, Nationwide and Santander.
The smaller building societies were adjudged to have 'low or no likelihood of support'.
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