Standard & Poor’s Ratings Services warned Tuesday morning of a coming slide in commercial mortgage-backed securities, as the economic recession appears set to take a bite out of one of the few remaining real estate asset classes to survive much of the turmoil in financial markets worldwide. The rating agency said it had recently performed a loan-by-loan, deal-by-deal summary of CMBS it had rated, factoring in current and expected economic conditions; the analysis found a “high downgrade risk” for recent-vintage CMBS, the firm said.