US regulators have warned Standard & Poor’s that they may file civil charges against the credit rating firm, alleging that it violated federal securities laws in connection with its rating of one of the structured finance instruments that was at the heart of the financial crisis.

McGraw-Hill, which owns S&P, said it had received a so-called Wells notice from the Securities and Exchange Commission on September 22 concerning its rating of a $1.6bn deal known as Delphinus CDO 2007-1. Such collateralised debt obligations bundle together debts and slice them into tranches of varying return and risk.

The Wells notice marks the first time the SEC has sought to pursue charges against a ratings company in connection with its rating of a CDO, linked to pools of residential mortgages.