Trading in credit derivatives has surged in recent weeks, partly because investors have tried to protect themselves from falling bond prices after events such as the ratings downgrades for General Motors, bankers say.

The scale of the business is impossible to quantify, because the sector is so new that trading occurs off public exchanges, making it extremely opaque. But bankers report a sharp increase both in trading for instruments, such as the European Itraxx indices (indices of credit derivatives), and for derivatives on individual company names.