"If the Thai sovereign rating and the Thai country ceiling were to be lowered, this would lead to a lowering of the following four banks' ratings given that their ratings are linked to the sovereign's [Krung Thai Bank and Export Import Bank of Thailand] or that they are foreign-owned banks that have ratings constrained by the country ceiling [United Overseas Bank (Thai) and Standard Chartered Bank (Thai)]," Fitch said in a statement.

Fitch has also placed the ratings of three other major Thai banks - Bangkok Bank, Kasikornbank and Siam Commercial Bank - on "Rating Watch Negative" pending a resolution of the political crisis that triggered the coup. If it is resolved quickly and confidence is restored after the past several months of political turmoil, the implications for the Thai economy and for the banks could even be positive. But if the crisis is not resolved quickly and peacefully, the negative effects on the economy and ultimately on the banks, could be significant.