The big rating agencies have agreed to change the way they charge for credit ratings in the mortgage-backed securities market to prevent 'ratings shopping', where only the highest ratings on a deal are selected and paid for.

The agreement, reached with a view to ending an investigation by Andrew Cuomo, New York’s attorney general, of Fitch Ratings, Moody’s Investors Service and Standard & Poor’s, is expected to be announced this week, people familiar with the deal said.

Rating agencies, which are paid by the issuers whose securities they rate, have come under criticism for failing to act quickly enough to warn investors about the complex debt and mortgagerelated products at the heart of the credit crisis. Many bonds with triple A credit ratings, which indicates the lowest risk of default, have been downgraded to much lower ratings, resulting in sharp losses on the bonds.

Financial Times