WASHINGTON -- The White House is reviewing a proposed finding by the Environmental Protection Agency that global warming is a threat to public health and welfare.

Such a declaration would be the first step to regulating carbon dioxide and other greenhouse gases under the Clean Air Act and could have broad economic and environmental ramifications. It also would likely spur action by Congress to address climate change more broadly.

The White House acknowledged Monday that the EPA had transmitted its proposed finding on global warming to the Office of Management and Budget, but provided no details. It also cautioned that the Obama administration, which sees responding to climate change a top priority, nevertheless is ready to move cautiously when it comes to actually regulating greenhouse gases, preferring to have Congress act on the matter.

The Supreme Court two years ago directed the EPA to decide whether greenhouse gases, especially carbon dioxide from burning fossil fuels, pose a threat public health and welfare because they are warming the earth. If such a finding is made, these emissions are required to be regulated under the Clean Air Act, the court said.

"I think this is just the step in that process," said White House Press Secretary Robert Gibbs, noting the Supreme Court ruling. Another White House official, speaking anonymously in deference to Gibbs, predicted "a long process" before any rules would be expected to be issued on heat-trapping emissions.

But several congressional officials, also speaking on condition of anonymity because the draft declaration had not been made public, said the transmission makes clear the EPA is moving to declare carbon dioxide and other greenhouse gases a danger to public health and welfare and views them as ripe for regulation under the Clean Air Act.

Such a finding "will officially end the era of denial on global warming," said Rep. Ed Markey, D-Mass., whose Energy and Commerce subcommittee is crafting global warming legislation. He said such an endangerment finding is long overdue because of the Bush administration's refusal to address the issue.

The EPA action "signals that the days of ignoring this pressing issue are over," said Sen. Barbara Boxer, D-Calif., whose Senate committee is working on a climate bill.

Many business leaders argue, as did President Bush, that the Clean Air Act is ill suited to deal with climate change and that regulating carbon dioxide would hamstring economic growth.

"It will require a huge cascade of (new clean air) permits" and halt a wide array of projects, from building coal plants to highway construction, including many at the heart of President Barack Obama's economic recovery plan, said Bill Kovacs, a vice president for environmental and technology issues at the U.S. Chamber of Commerce.

Abigail Dillen, an attorney for the environmental advocacy group Earthjustice, which is involved in a number of lawsuits challenging permits for new coal plants, dismissed the dire economic warnings from business groups about carbon dioxide regulation.

"It's to their interest to say the sky is falling, but it's not. ... The truth is we've never had to sacrifice air quality to maintain a healthy economy. The EPA has discretion to do this in a reasonable way."

An internal EPA planning document that surfaced recently suggests the agency would like to have a final endangerment finding by mid-April. But officials have made clear actual regulations are unlikely to come immediately and involve a lengthy process with public comment.

Gibbs, when asked about the EPA document Monday, emphasized that "the president has made quite clear" that he prefers to have the climate issue addressed by Congress as part of a broad, mandatory limit on heat-trapping emissions.

But environmentalists said the significance of moving forward with the long-delayed endangerment issue should not be understated.

"This is historic news," said Frank O'Donnell who heads Clean Air Watch, an advocacy group. "It will set the stage for the first-ever national limits on global warming pollution and is likely to help light a fire under Congress to get moving."