Standard & Poor's cut the outlook for South African ratings to negative from stable on Tuesday on concerns about a large current account deficit and slowing world growth.
The revision follows a similar move by Fitch on Monday, which was criticised by the country's Treasury.
"The outlook revision reflects pressures on South Africa's balance of payments, which increase the risk of further currency depreciation and a sharper-than-anticipated correction in the current account deficit," S&P credit analyst Remy Walters said in a statement.
The ratings agency affirmed South Africa's "BBB+/A-2" foreign currency and "A+/A-1" local currency ratings.
It said local banks had limited exposure to the global financial crisis and should cope with an increase in bad debts as households struggle to cope with high interest rates and inflation.
But the economy would feel the impact of the global slowdown.
The rand had depreciated sharply this year and net portfolio flows were likely to remain negative, putting further pressure on the currency.
This would add to inflationary pressures and delay an expected easing in monetary policy at a time when economic growth was easing, S&P said.
The rand has weakened by around 30% against the dollar in 2008.
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