Her resignation was to take effect June 27, according to a copy of the May 20 letter posted on the EPA website.

"With gratitude for the opportunity to serve the American people in your administration," she wrote to Bush, "I hereby tender my resignation as administrator of the US Environmental Protection Agency, effective June 27,

"It has been a singular honor to be entrusted with the responsibility to lead the EPA in its effort to leave America's air cleaner, its water purer and its land better protected than it was when this administration took office," Whitman wrote.

Whitman, 56, who previously served as governor of the state of New Jersey, took the reins at the EPA in January 2001, as Bush entered the White House. She was known as one of the more moderate members of the Bush administration.

Still, early on, she had to confront opposition from environmentalists over the administration's decision not to ratify the Kyoto Protocol on global warming, designed to reduce greenhouse gas emissions.

Other controversial moves likewise drew environmentalists' ire, including the relaxing of rules governing factory pollution and the development of the so-called Healthy Forests Initiative, aimed at thinning US forests in a bid to protect against wildfires.

The legislation was adopted by the House of Representatives Tuesday and must now be passed by the Senate and signed by Bush in order to take effect.

"Christie Todd has been a dedicated and tireless fighter for new and innovative policies for cleaner air, purer water, and better protected land," the president said in a statement.

"I thank her for her outstanding service to our Nation and wish her well as she returns home to New Jersey."

Gary Gardner, director of research for the Worldwatch Institute in Washington, told AFP he was "not surprised" by Whitman's resignation.

"I'm actually a little surprised that it took as long as it did," he said.

Given her record in New Jersey, "where she made an effort on the environmental front, it was very surprising to see her reign over the wholesale dismantlement of 30 years of environmental legislation and regulation in the United States," Gardner said.

Whitman saw the administration's environmental record in a much more favorable light, however.

"Our work has been guided by the strong belief that environmental protection and economic prosperity can and must go hand in hand, that the true measure of the value of any environmental policy is in the environmental results it produces," she wrote in her letter.

"I am pleased that the EPA has built an enviable record of success that will result in significant improvements to the state of our nation's treasured environment."

She said she would be returning to New Jersey.

"As rewarding as the past two-and-a-half years have been for me professionally, it is time to return to my home and husband in New Jersey," the mother of two wrote, adding: "I leave knowing that we have made a positive difference and that we have set the agency on a course that will result in continued environmental improvement."

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