Scott Pruitt will resign from his position leading the Environmental Protection Agency (EPA) on Friday, following months of high-profile controversies regarding his spending, ethics and management at the agency.

In a tweet Thursday, President Trump confirmed Pruitt's departure, saying he's accepted the administrator's resignation.

"I have accepted the resignation of Scott Pruitt as the Administrator of the Environmental Protection Agency. Within the Agency Scott has done an outstanding job, and I will always be thankful to him for this," Trump tweeted.

Trump said Pruitt will be replaced on Monday by EPA Deputy Administrator Andrew Wheeler, a former energy lobbyist who was confirmed in April for the post.

"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!"

In comments on Air Force One, Trump said there was no "final straw" that explained Pruitt's departure. He said the Cabinet member had indicated he did not want to be a "distraction."

"He'll go on to great things and he's going to have a wonderful life, I hope. But he felt that he did not want to be a distraction for an administration that he has a lot of faith in," Trump said.

He said the decision has been in the works for "a couple of days."

"We've been talking about it for a little while," Trump said.

In his resignation letter to President Trump on Thursday, Pruitt cited "unrelenting attacks" as his reason to leave the agency.

"It is extremely difficult for me to cease serving you in this role first because I count it a blessing to be serving you in any capacity, but also, because of the transformative work that is occurring," Pruitt wrote Trump. "However, the unrelenting attacks on me personally, my family, are unprecedented and have taken a sizable toll on all of us."

Pruitt signed his resignation letter to Trump: "Your Faithful Friend."

Pruitt this week faced intensified criticism from reports that the administrator maintained a secret, internal calendar that he directed aides to change from the public version they released.

Pruitt's former political aide, Madeline Morris, confirmed to the New York Times Thursday that she believes she was fired from the agency after pushing back on directions to retroactively change meetings on Pruitt's calendar - actions she learned were potentially illegal.

Morris was hired in June 2017 as Pruitt scheduler. She raised red flags after seeing changes related to Pruitt's trip to Italy on the calendar. She told the Times she was fired two days later in August 2017.

Former EPA political staffer turned whistleblower Kevin Chmielewski confirmed to The Hill that he and Pruitt's Chief of Staff Ryan Jackson were responsible for firing Morris for the changes.

Chmielewski said it was typical of staffers to be "retaliated against and assigned."

He said he was fired from the agency earlier this year after he pushed back on a number of Pruitt's travel plans including use of first class travel.

Chmielewski additionally confirmed the use of an internal calendar he said staffers kept out of the public eye. CNN first reported on the secret calendar Tuesday.

Pruitt has been engulfed in high-profile scandals and under fire by both parties in recent months regarding spending taxpayer money for travel and security, his close relationships with lobbyists and industry, and allegations he used government resources and staff for personal gain, among other controversies.

He was facing more than 13 federal investigations over the controversies, including from the EPA's Office of Inspector General, the U.S. Office of Special Counsel and the House Oversight and Government Reform Committee.

The Government Accountability Office in April found him guilty of overspending congressionally allocated money on a $43,000 soundproof booth built in his office to provide privacy on personal calls.

Despite a nearly circuitous trend of scandals surrounding Pruitt at EPA, the administrator appeared to walk away from most criticisms unscathed.

Trump expressed confidence in the embattled EPA chief as recently as last month.

"Scott Pruitt is doing a great job within the walls of the EPA. I mean, we're setting records," Trump said June 8, adding, "I'm not saying that he's blameless, but we'll see what happens."

But Pruitt had been losing the confidence of top White House officials and career staffers at the agency, and many of his closest aides left as the scandals grew.

"The president feels as though Scott Pruitt has done a really good job with deregulating the government, to allow for a thriving economy, that's important to him, but these things matter to the president as well, and he's looking into those," deputy White House press secretary Hogan Gidley told reporters July 3.

While the EPA head has been under nearly constant criticism by Democrats and environmentalists, the most serious of his scandals started earlier this year, with revelations that he flew first-class nearly exclusive while traveling on the taxpayers' dime - costing at least $105,000 in his first year on the job - and that he had paid just $50 for each night he spent at a Capitol Hill condo owned by Vicki Hart, whose husband, J. Steven Hart, was chairman of lobbying firm Williams & Jensen.

Pruitt's close dealings with lobbyists didn't stop there. Industry advocates were largely responsible for planning some of Pruitt's proposed and actual international trips, including one to Morocco and a canceled one to Australia.

The EPA also gave big raises two top Pruitt aides in March, using a special authority under a 1970s law after the White House refused to allow it. Pruitt told lawmakers in April that he knew about the raises, but did not know his chief of staff Ryan Jackson would go around the White House to implement them.

Pruitt has long been under fire for spending taxpayer money. He also doled out $43,000 for a soundproof booth for his office, had a round-the-clock security force costing $3.5 million in his first year and had the agency get him a new SUV and bulletproof vests, all on the taxpayers' dime.

The Government Accountability Office ruled that the soundproof booth expenditure was illegal, because the EPA did not notify Congress that it would exceed a $5,000 cap on furnishing officials' offices.

In more recent months, the scandals have focused on allegations that he used EPA staff and his security detail for personal reasons, like shopping for apartments, finding jobs for his wife, buying a used Trump International Hotel mattress and tracking down a lotion he wanted from a Ritz-Carlton hotel.

Pruitt has defended much of the security spending as a necessary step, saying that he has faced an unprecedented number of threats, a contention that lawmakers have disputed.

As for other scandals, Pruitt has largely blamed the missteps on staff, though he conceded he may have been able to do more to stop them.

"Some of the areas of criticism are frankly areas where processes at the agency were not properly instituted to prevent certain abuses from happen," he said in a May 16 Senate hearing. "There have been decisions over the last 16 months or so that, as I look back on those decisions, I would not make the same decisions again."

Pruitt has also been combative in defending himself, frequently blaming the scandals on the political left and the media.

Pruitt has consistently had allies among congressional Republicans and conservative commentators who have cheered his deregulatory actions, but even that has faltered.

"He's hurting the president because he has, I'm sorry, bad judgment after bad judgment after bad judgment," conservative host Laura Ingraham said on her radio show in June. "If you want to drain the swamp, you've got to have people in it who forgo personal benefits, and don't send your aides around doing personal errands on the taxpayer dime. Otherwise, you make everybody else look bad."

Pruitt, 50, had led the EPA since the Senate confirmed him 52-46, mostly along party lines, in February 2017. He was previously Oklahoma's attorney general and made a name for himself for filing high-profile lawsuits against Obama administration policies, including more than a dozen against the agency he went on to lead.

Pruitt has been one of the most polarizing figures in the Trump administration, detested by the left for his aggressive environmental deregulatory agenda and embraced on the right.

He worked quickly to make changes at the agency during his 17 months in office, having taken dozens of actions to roll back or change major Obama administration and other policies including the Clean Power Plan, Clean Water Rule and methane standards for oil and natural gas drillers, and had been seen as being in good favor with Trump.

An EPA spokesperson earlier Thursday defended Pruitt's scandals by highlighting those same regulatory rollbacks.

"From advocating to leave the Paris Accord, working to repeal Obama's Clean Power Plan and Waters of the United States, declaring a war on lead and cleaning up toxic Superfund sites, Administrator Pruitt is focused on advancing President Trump's agenda of regulatory certainty and environmental stewardship spokesperson," the spokesperson said in a statement to The Hill.

Pruitt announced amid the scandals that he would seek to ease greenhouse gas emissions standards for cars set by the Obama administration, endorsing the auto industry's argument that they are unattainable because consumers are buying bigger cars. He also rolled out a controversial science "transparency" initiative, proposing to restrict the agency from using scientific findings whose methodology and data are not sufficiently public or reproducible.

Supporters often said Pruitt was one of the most effective Cabinet members in the Trump administration. And opponents said Pruitt was very active and aggressive in his deregulatory mission, but often ran afoul of the courts.

Wheeler had long been seen by both supporters and opponents of Trump has a Pruitt replacement who could accomplish the same regulatory goals without the scandals. That will be put to the test starting Monday.

"There's a guy behind him, Andrew Wheeler, who's really qualified too. So that might be a good swap," Sen. Jim Inhofe (R-Okla.), for whom Wheeler used to work, told Ingraham in June.

Wheeler is in line with most of the GOP's priorities for the EPA, including removing what they see as barriers to industries like oil, gas and coal.

However, Wheeler demurred when it came to suggestions that he was primed to take over for Pruitt, telling the Hill as recently as last week in an interview that he wasn't interested in Pruitt's job.

"I'm the deputy administrator, that's the position I signed up for, that's the position I wanted. I didn't want to be the administrator, still don't want to be the administrator," he said.

"I'm here to help Administrator Pruitt with his agenda and President Trump's agenda for the agency. That's what my job is."

Updated: 5:45 p.m.