The relative credit-worthiness of Santander UK - the country's fifth biggest High Street bank - has been questioned by one of the main financial credit rating agencies, Moody's.
It has downgraded the UK bank along with the credit rating of 16 Spanish lenders, in the light of the huge debts of that country's banking system.
The downgrades included Santander's parent, Banco Santander, one of Spain's biggest banks.
Last week, the Spanish government had to partially nationalise Bankia, the country's fourth biggest lender, to stop it being tipped into insolvency by its massive losses on past property loans.
It has also had to ask its eurozone partners for billions of euros in loans to help its banking sector.