WASHINGTON — The Environmental Protection Agency on Monday issued a final ruling that greenhouse gases posed a danger to human health and the environment, paving the way for regulation of carbon dioxide emissions from vehicles, power plants, factories, refineries and other major sources.

The announcement was timed to coincide with the opening of the United Nations conference on climate change in Copenhagen, strengthening President Obama’s hand as more than 190 nations struggle to reach a global accord.

The E.P.A.’s administrator, Lisa P. Jackson, said that a 2007 decision by the Supreme Court required the agency to weigh whether carbon dioxide and five other climate-altering gases threatened human health and welfare and, if so, to take steps to regulate them.

She said Monday that the finding was driven by the weight of scientific evidence that the planet was warming and that human activity was largely responsible.

“There have and continue to be debates about how and how quickly climate change will happen if we fail to act,” Ms. Jackson said at a news conference at the E.P.A.’s headquarters. “But the overwhelming amounts of scientific study show that the threat is real.”

Industry groups quickly criticized the decision, saying that the regulation of carbon dioxide, a near-ubiquitous substance, would be legally and technically complex and would impose huge costs across the economy.

In her prepared remarks and in response to questions, Ms. Jackson waded into the current dispute over leaked e-mail messages from a British climate research group that have stirred doubts among a number of people about the integrity of some climate science.

Several Republicans in Congress had asked the E.P.A. to delay the so-called endangerment finding because of questions about the underlying science. Ms. Jackson rejected their plea.

“We know that skeptics have and will continue to try to sow doubts about the science,” she said. “It’s no wonder that many people are confused. But raising doubts — even in the face of overwhelming evidence — is a tactic that has been used by defenders of the status quo for years.”

She said that the agency had reviewed the arguments of some of those skeptics during months of public comment but that none of them had raised significant new issues.

The Obama administration had signaled its intent to issue an endangerment finding for carbon dioxide and five other greenhouse gases (methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride) since taking office in January. Ms. Jackson announced a proposed finding in April and has since taken steps to draft the rules needed to back it up.

The administration has used the finding as a prod to Congress, saying that if lawmakers do not act to control greenhouse gas pollution it will use its rule-making power to do so. At the same time, the president and his top environmental aides have said that they prefer such a major step be taken through the legislative process.

The administration struck a deal with automakers last spring to set stricter tailpipe emissions and higher fuel economy standards as part of the greenhouse gas regulation efforts. The E.P.A. has also announced rules requiring all major emitters to report an annual inventory of emissions.

In late September, the agency announced a proposed “tailoring rule” that limits regulation of climate-altering gases to large stationary sources like coal-burning power plants and cement kilns that produce 25,000 tons or more a year of carbon emissions.

Industry groups said that the finding and the proposed regulations would damage the economy and drive jobs overseas. Some groups are likely to file lawsuits challenging the new regulations, which could delay their effective date for some years.

“E.P.A. is moving forward with an agenda that will put additional burdens on manufacturers, cost jobs and drive up the price of energy,” said Keith McCoy, vice president of energy policy at the National Association of Manufacturers.

“Unemployment is hovering at 10 percent, and many manufacturers are struggling to stay in business,” he said. “It is doubtful that the endangerment finding will achieve its stated goal, but it is certain to come at a huge cost to the economy.”

Jeff Holmstead, head of air policy at the E.P.A. under the administration of George W. Bush and now an industry lobbyist, said the finding was mainly symbolic.

“It does not have any immediate effect and does not impose any regulations or requirements on anyone,” he said. “Today’s announcement comes as no surprise and is clearly designed to set the stage for the Copenhagen conference.”

Environmental advocates who have pushed for the finding for years exulted.

“The stage is now set for E.P.A. to hold the biggest global-warming polluters accountable,” said Emily Figdor, federal global-warming program director for Environment America, an activist group.