Ratings agency Moody's on Friday lowered its outlook for the South African banking system to negative from stable, citing deteriorating operating conditions over the next 18 months, sending shares across the sector lower.
Africa's most advanced economy is expected to grow less than 1 percent this year, hobbled by low commodity prices, drought and political ructions that have unnerved investors.
As of 1338 GMT shares in Barclays Africa were down 2.2 percent, Standard Bank had shed 1.08 percent and Investec was 0.5 percent lower.
"The outlook expresses Moody's expectation of how bank creditworthiness will evolve in this system over the next 12 to 18 months," the annual banking system outlook said.
"The challenging economic outlook will strain borrowers' repayment capacity, fuelling increased asset risks."
Moody's earlier this month kept South Africa's sovereign rating on hold at Baa2 with a negative outlook, two notches above junk.
The ratings agency said it expects the banking system's non-performing loan ratio to rise to around 4 percent by the end of 2017 from 3.1 percent in December 2015 due to pressure on corporates and consumers from rising interest rates and inflation.
It also said profitability in the sector could come under strain due to waning demand for credit and lower business opportunities.
Fellow ratings agency Fitch, which has South Africa at one notch above speculative grade, said on Thursday the country's authorities should avoid populist measures such as introducing a minimum wage in the run up to local elections in August.
Standard & Poor's also rates South Africa's debt at BBB-, one notch above speculative grade and with a negative outlook.
"We have expected the lower outlook for a while," said Graeme Korner, director at fund manager Korner Perspective, which invests in banks.
"We think the earnings of the banks will be OK in spite of these concerns because the fact is South African banks are very good value and pay good dividends."
(Reporting by Zandi Shabalala; Editing by Hugh Lawson)