Big banks are awarded higher grades from ratings firms becausethe lenders provide them with business including evaluatingsecuritized debt, according to a European Central Bank study.

Larger financial institutions were more likely to receive bettergrades, according to the research report, which reviewed about39,000 quarterly bank ratings from Standard & Poor's, Moody'sInvestors Service and Fitch Ratings from 1990 to 2011.

Inflated grades on bonds backed by subprime mortgages during thehousing boom helped ignite the worst financial crisis since theGreat Depression when their values plummeted five years ago.Analysts at the three firms were pressured to give their stamp ofapproval to complex investments to win lucrative business from WallStreet banks, the Senate Permanent Subcommittee on Investigationssaid last year in a report.