The U.S. Securities and Exchange Commission agreed to back an effort by banks that may delay writedowns on some securities tied to losses that have cost companies more than $640 billion.

Banks in certain cases may account for perpetual preferred securities as debt, allowing them to postpone writing down their value, SEC Chief Accountant Conrad Hewitt wrote in a letter yesterday to Financial Accounting Standards Board Chairman Robert Herz. Hewitt’s interpretation can be used immediately, as companies prepare quarterly financial reports.