Moody's has downgraded the debt of four major US banks, including JP Morgan and Goldman Sachs, in a move that could signal the beginning of the end of an era when the country's largest financial institutions have generally been considered too systemically important to be allowed to fail.

In an acknowledgement of the US government's efforts to fortify the country's financial system and put in place a clear framework for unwinding failed banks – meant to ensure orderly resolution but also to limit