Moody's Investors Service has downgraded the credit rating on 28 Spanish banks, including Banco Santander, less than a month after cutting the nation's sovereign rating.
The ratings agency said the banks are linked to the sovereign, so Spain's reduced creditworthiness "implies a weaker credit profile for Spanish banks".
The move follows a cut to the Spanish government's own credit rating to just above junk status earlier in June.
Among the downgrades was a cut to Banco Santander, which saw its long-term rating cut to Baa2 from A3. The bank's UK arm is a standalone business.
On Monday, Spain asked for money to help support its banks after an independent audit last week found they needed up to 62bn euros (£50bn) in extra funding.
European authorities had already agreed to provide up to 100bn euros ahead of assessments of the banks' needs.
Service increasingly key
Aiming to be' top three' UK financial planner
Lowest measure since index launched in 1995
Complaints into double figures
Despite lower median annual earnings