Moody's said shareholder and political pressure was creating uncertainty about the bank's future.
And it added: "Although this could have potentially positive implications over the longer term, the uncertainty surrounding such a change in direction is credit negative in the short term."
Meanwhile S&P said the emergence of "weak business practices" and "current management flux" had hit the company's prospects.
This week has been turbulent for the lender after chairman Marcus Agius, chief executive Bob Diamond and chief operating officer Jerry del Missier all quit the bank.
Following Diamond's evidence session with the Treasury Select Committee yesterday MPs are due to vote on whether to have a parliamentary or independent inquiry into the LIBOR fixing scandal which affects up to 15 other banks.
