By John Whitesides

WASHINGTON (Reuters) - U.S. Environmental Protection Agency chief Scott Pruitt, who had been lauded by President Donald Trump for his aggressive efforts to roll back environmental regulations, resigned on Thursday under heavy fire for a series of ethics-related controversies.

Pruitt was one of Trump's most polarizing Cabinet members, slashing regulations on the energy and manufacturing industries, including a move to repeal former President Barack Obama's signature program to cut carbon emissions from power plants, dubbed the Clean Power Plan.

He was also instrumental last year in lobbying Trump to withdraw the United States from the global 2015 Paris climate accord to combat global warming.

But Pruitt lost favor with Trump's inner circle after a string of controversies including first-class travel at taxpayer expense, lavish spending on security, the installation of a $43,000 soundproof phone booth in his office and accusations he used his position to receive favors, such as a discounted rental on a high-end condo from an energy lobbyist's wife.

"The unrelenting attacks on me personally, my family, are unprecedented and have taken a sizable toll on all of us," Pruitt said in his resignation letter.

Trump announced the resignation on Twitter and said EPA Deputy Administrator Andrew Wheeler, a former mining industry lobbyist, will become the regulatory agency's acting chief on Monday. Wheeler is widely expected to continue Pruitt's efforts to roll back and streamline regulation.

"Scott has done an outstanding job, and I will always be thankful to him for this," Trump wrote. Trump told reporters later that Pruitt had approached him and offered to resign as opposed to being pushed out.

Democrats and environmental advocacy groups cheered the departure of Pruitt, a close ally of the fossil fuel industry who has often questioned mainstream climate change science.

"Scott Pruitt's reign of venality is finally over. He made swamp creatures blush with his shameless excesses. All tolerated because Trump liked his zealotry. Shame," Democratic U.S. Representative Gerry Connolly said.

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The Environmental Working Group, a public health and environment watchdog, called Pruitt "unquestionably the worst head of the agency in its 48-year history."

Pruitt also rankled some Republican lawmakers, including in Midwest corn-producing states, with his efforts to overhaul a U.S. policy requiring biofuels like corn-based ethanol in gasoline.

Republican Senator Chuck Grassley of Iowa, said Trump made the "right decision."

Other Republicans, as well as coal and oil industry groups, issued statements on Thursday saying Pruitt had been a good friend to industry.

"Scott Pruitt did great work to reduce the regulatory burdens facing our nation while leading the Environmental Protection Agency," said Republican Senator Jim Inhofe, from Pruitt's home state of Oklahoma.

'POLICY WILL REMAIN'

Pruitt’s interim replacement, Wheeler, was a former lobbyist for Murray Energy [MUYEY.UL], the country’s largest underground coal mining company, and also worked for Inhofe – a self-described climate skeptic - on efforts to combat climate legislation.

Matt Dempsey, an energy lobbyist at consultancy FTI, said Wheeler will be less controversial than Pruitt, but without altering the agenda.

"He will be less political and more straightforward in his approach to the job, which is better for the Trump administration agenda in the long run. The politics will pass but the policy will remain," he said.

That alarmed some critics of the agency's policies under the Trump administration.

Michael Mikulka, president of the American Federation of Government Employees Local 704, which represents some EPA workers in the Midwest, said Wheeler "has the potential to be just as destructive."

"We urge him to immediately commit to undoing the massive damage to EPA’s work done under Pruitt's leadership," he said.

Pruitt was facing around a dozen investigations into his tenure, including his frequent use of first-class flights and his spending on security – which the agency has defended as necessary to defend him against unprecedented threats.

Travel records showed the U.S. government spent $17,000 in taxpayer money on a December trip to Morocco to promote U.S. exports of liquefied natural gas, which is not part of the EPA's jurisdiction. The Washington Post reported that a longtime Pruitt friend and lobbyist helped arrange the trip and later registered as a foreign agent representing Morocco.

In one of the investigations, the U.S. Government Accountability Office concluded the EPA violated two laws by installing the $43,000 phone booth for his office without telling lawmakers first. Pruitt said his staff never told him the cost.

Some of the ethics accusations against Pruitt also involved jobs for his wife. Emails obtained by the Sierra Club environmental group showed Pruitt had an aide contact the chief executive of a fast-food chain about his wife becoming a franchise owner.

The Washington Post reported Pruitt had aides also try to get her a job at the Republican Attorneys General Association with a salary topping $200,000 per year.

Pruitt also had an employee carry out his personal errands, including researching the purchase of an old mattress from the Trump International Hotel, according to an interview transcript released by congressional Democrats last month.

A source close to Trump said it was a controversy over the search for a used Trump International Hotel mattress that was the last straw for Trump with Pruitt because it involved the Trump Organization.

During congressional testimony in April, Pruitt was unapologetic for the controversies, often blaming his staff for any agency missteps.

Pruitt was also known for questioning the human role in global climate change. As Oklahoma's former attorney general, he waged a legal fight against environmental rules implemented by the agency he eventually would head.

(Reporting by Valerie Volcovici, Eric Walsh, Eric Beech and Makini Brice, James Oliphant and Steve Holland; writing by John Whitesides; editing by Will Dunham, Tom Brown and G Crosse)