JPMorgan Chase & Co. and Bank of America Corp. are helping clients find an extra $2.6 trillion to back derivatives trades amid signs that a shortage of quality collateral will erode efforts to safeguard the financial system.

Starting next year, new rules designed to prevent another meltdown will force traders to post U.S. Treasury bonds or other top-rated holdings to guarantee more of their bets. The change takes effect as the $10.8 trillion market for Treasuries is already stretched thin by banks rebuilding balance sheets and investors seeking safety, leaving fewer bonds available to backstop the $648 trillion derivatives market.

The solution: At least seven banks plan to let customers swap lower-rated securities that don't meet standards in return for a loan of Treasuries or similar holdings that do qualify, a process dubbed "collateral transformation." That's raising concerns among investors, bank executives and academics that measures intended to avert risk are hiding it instead.