The rating agency confirmed the island's 'B' long-term foreign currency rating, its 'B+' local currency rating and 'B' short-term rating.
The rating agency said information on foreign exchange reserves showed a 26.8 percent fall in gross official reserves since the beginning of the year, even as the trade deficit narrowed.
S & P said a further fall in external liquidity or delays in getting an IMF loan would trigger a rating downgrade.
If an IMF loan is given and Sri Lanka reaches the program targets, the rating would remain steady.
"Standard & Poor's believes the military defeat of the separatist LTTE will not alleviate near-term pressure on external liquidity, nor is it likely to reverse the countr