In a major blow to the already struggling world banking sector, rating agency major Standard and Poor – S&P – has downgraded as many as 15 large banks globally.These include some big American names like Bank of America, Citigroup and Goldman Sachs.It has also cut ratings for US-based banking giants like JP Morgan, Morgan Stanley, Wells Fargo, while some European banking titans like Barclays, HSBC, Lloyds Banking Group, Royal Bank of Scotland and Rabobank have also been downgraded.In a rating review of as many as 37 banks globally, S&P has incidentally upgraded two Chinese banks – Bank of China and China Construction Bank.At the same time, it has kept its ratings unchanged for a handful of big European names like BNP Paribas, Credit Agricole and Deutsche Bank.The review did not include any Indian banks.Another agency Moody's has also hinted at possible rating downgrades for some European banks.