Three of Wall Street's biggest investment banks are set to announce today that they are imposing new environmental standards that will make it harder for companies to get financing to build coal-fired power plants in the U.S.

Citigroup Inc., J.P. Morgan Chase & Co. and Morgan Stanley say they have concluded that the U.S. government will cap greenhouse-gas emissions from power plants sometime in the next few years. The banks will require utilities seeking financing for plants before then to prove the plants will be economically...