Fitch Credit Ratings downgraded the credit default ratings of 18 Spanish banks Tuesday, following the downgrade of the country's sovereign debt last week.

The downgrade of Spanish sovereign debt was factored into the decision to downgrade the banks, particularly those with heavy exposure to construction and real estate and those with low equity, Fitch said.

The banks affected by the downgrade are all domestic banks whose revenue generation and risk profiles are highly sensitive to the evolution of Spain's economy and its housing market, the credit rating agency said.