The standards do not preclude bank financing for building traditional coal-burning power plants, said Jeffrey Holzschuh, vice chairman at Morgan Stanley. Instead, they set up a more rigorous evaluation process, such as looking at the costs of storing carbon emissions and other risk factors.

The principles are intended to be an industry-wide framework with more financial institutions jumping on board.

"I think there will be several other banks that will join in over the next few weeks," Holzschuh told Reuters.

Coal generates about half of US electricity, but is the dirtiest emitter of the main greenhouse gas carbon dioxide. Traditional coal-fired plants have come under pressure from states and environmentalists, while the US Congress considers several bills that would cap greenhouse gas emissions. Presidential candidates also say they favor regulating the gases.

Plans for new coal-fired plants have been scuttled recently in Texas, Florida and Kansas as environmental groups work with banks to highlight their emissions risks. But dozens more of the plants are in various stages of planning as the government predicts US power demand to grow steadily in coming decades.

"The days of conventional coal are over," Mark Brownstein of Environmental Defense, one of the groups involved with helping form the framework, told Reuters.

The three banks developed the principles in consultation with environmental organizations and power companies, including American Electric Power Co, the nation's largest consumer of coal, and Southern Co, the largest utility company in the coal-heavy Southeast.

"Having just come from the subprime mess, financial institutions are taking a second look at their risk management practices," Brownstein told Reuters.

"Taking into account future CO2 liabilities is the way to make sure that the investments you make today don't come back to bite you tomorrow,"

When assessing new coal-plant financing, the principles will also look at energy efficiency, advanced cleaner-coal technology that emit less carbon, and renewable sources of power.