“The unrelenting attacks on me personally, my family, are unprecedented and have taken a sizeable 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 offered to resign and was not pushed out.

Democrats and environmental advocacy groups cheered the departure of Pruitt, a close ally of the fossil fuel industry who has questioned claims about climate change being man-made.

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

Some 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.

Pruitt’s interim replacement, Wheeler, was a former lobbyist for Murray Energy, the country’s largest underground coal mining company, and also worked for Inhofe – a self-described man-made climate change sceptic – 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 said was necessary to defend him against unprecedented threats.

Travel records showed the US government spent US$17,000 in taxpayer money on a trip to Morocco to promote US exports of liquefied natural gas, which is not part of the EPA’s jurisdiction. The Washington Post said a long-time Pruitt friend and lobbyist helped arrange the trip and later registered as a foreign agent representing Morocco.

In one of the investigations, the US Government Accountability Office concluded the EPA violated two laws by installing the US$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 that Pruitt’s aides also tried to get her a job at the Republican Attorneys General Association with a salary topping US$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 that was the last straw for Trump because it involved the Trump Organisation.

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