Donald Trump said that he has accepted the resignation of Environmental Protection Agency chief Scott Pruitt, whose tenure has been mired in controversy over spending he oversaw at the agency.
The president claimed in a tweet that Mr Pruitt had "done an outstanding job, and I will always be thankful to him for this."
He added: "The Senate confirmed Deputy at EPA, Andrew Wheeler, will on Monday assume duties as the acting Administrator of the EPA. I have no doubt that Andy will continue on with our great and lasting EPA agenda. We have made tremendous progress and the future of the EPA is very bright!"
The unexpected announcement came after a steady stream of negative headlines about Mr Pruitt in recent months.
Questions have been raid about Mr Pruitt's travel costs, which have included first-class seats on jetliners, chartered private planes, and a $20,000 (£15,000) four-day trip to Morocco.
He has also been criticised for enlisting government aides to try to find a job for his wife with the Republican Attorneys General Association and setting them with other menial personal tasks.
It also emerged that he was renting an apartment in Washington at a relatively cheap rate from a healthcare lobbyist whose husband had lobbied the EPA previously.
Mr Pruitt — who previously worked as the attorney general of Oklahoma, where he led a series of legal challenges against the EPA — has overseen an attack on the climate and environmental policies championed by Mr Trump's predecessor Barack Obama.
He has made the that the EPA's primary responsibility was to offer certainty to energy companies, automotive manufacturers, and other businesses that the agency regulates. Some of the agency's lead scientists have also been sidelined and he has attempted to ease regulations that the Obama administration pushed for to regulate greenhouse gasses.
Wildlife and conservation groups welcomed the resignation announcement.
"Wildlife should be dancing in the streets," Jamie Rappaport Clark, the president and CEO of Defenders of Wildlife, said in a statement. "Mr Pruitt was bad for the environment and bad for wildlife. He will not be missed".
Vermont Senator and former democratic presidential candidate Bernie Sanders, framed the resignation in even less flattering terms.
“Scott Pruitt was the worst EPA administrator in the history of the agency. Not only has he acted, time and time again, in an unethical manner, but he has led the agency in exactly the wrong direction. Instead of protecting our environment and combating climate change, he has worked to protect the interests of the fossil fuel industry and polluters all over the country. His resignation is a positive step forward for our country," Mr Sanders said in a statement. "I will do everything possible to see that the next EPA administrator actually believes in environmental protection.”
But it appears unlikely that he will be replaced by a champion of climate change mitigation — Mr Trump has at times suggested that climate change could be a good thing — and his temporary replacement, Mr Wheeler, is a former oil and gas lobbyist and former Senate staffer for Senator Jim Inhofe, where he attempted to exempt industrial plants from pollution controls and to shield polluters from liability over harm caused by toxic chemicals.
