Two of the main global credit rating agencies have downgraded some of Spain's biggest banks over fears for the Spanish economy.Standard & Poor's (S&P) said last night it had downgraded the ratings of 10 financial firms, including the country's two biggest banks, Santander and BBVA.Fitch said it was cutting the ratings of six banks. S&P said its latest action resulted from the "tougher-than-previously-anticipated macroeconomic and financial environment in Spain"."In our view, Spain's economy faces dimming growth prospects in the near term, real estate market activity remains depressed, and turbulence in the capital markets has heightened," the agency said.It predicted that problems with the Spanish economy would continue to cause problems for the banks for the next 15-18 months.S&P downgraded Banco Santander and BBBVA by one notch, to AA- from AA.Fitch also downgraded Santander one notch to AA-, and BBVA by one notch to A+.Santander was placed on negative outlook by the agencies.SB