LONDON (Reuters) - S&P Global completed the list of top credit rating agencies to warn that an "unprecedented" U.S. withdrawal from the World Bank and other top multilateral lenders would damage their prized triple-A credit ratings.

President Donald Trump signed an Executive Order last week for a six-month review of U.S. support to all international intergovernmental organisations to decide whether it should withdraw from them, or seek their reform.

S&P said the current triple-A ratings of the World Bank and other top development banks assume the U.S. remaining in place. Therefore, if Washington were to "limit" its support, any rating change that came as a result of that was likely to be "negative".

S&P's top MDB analyst Alexander Ekbom added that it would ultimately depend on the impact on the individual institution's capital and whether other major shareholders filled the void that would be left.

"Should the U.S. leave, which would be unprecedented, it is difficult to predict how any reallocation would be ironed out," he said.