WASHINGTON - Billionaire Tesla CEO Elon Musk is leaving the Trump administration after leading a tumultuous efficiency drive, during which he upended several federal agencies, but ultimately failed to deliver the generational savings he had sought.

His "off-boarding will begin tonight," a White House official told Reuters late Wednesday, confirming Musk's departure from government. Musk earlier on Wednesday took to his social media platform X to thank President Donald Trump as his time as a special government employee with the Department of Government Efficiency draws to an end.

His departure was quick and unceremonious. He did not have a formal conversation with Trump before announcing his exit, according to a source with knowledge of the matter, who added that his departure was decided "at a senior staff level."

ALSO READ: Report: Trump to shut down agency funding developing countries

While the precise circumstances of his exit were not immediately clear, he leaves a day after criticizing Trump's marquee tax bill, calling it too expensive and a measure that would undermine his work with the US DOGE Service.

Some senior White House officials, including Deputy Chief of Staff Stephen Miller, were particularly irked by those comments, and the White House was forced to call Republican senators to reiterate Trump's support for the package, a source familiar with the matter said.

While Musk remains close to the president, his exit comes after a gradual, but steady slide in standing.

After Trump's inauguration, the billionaire quickly emerged as a powerful force in Trump's orbit: hyper-visible, unapologetically brash and unfettered by traditional norms. At the Conservative Political Action Conference in February, he brandished a red metallic chainsaw to wild cheers. "This is the chainsaw for bureaucracy," he declared.

On the campaign trail, Musk had said DOGE would be able to cut at least $2 trillion in federal spending. He did not hide his animus for the federal workforce, and he predicted that revoking "the COVID-era privilege" of telework would trigger "a wave of voluntary terminations that we welcome."

But some cabinet members who initially embraced Musk's outsider energy grew wary of his tactics, sources said. Over time, they grew more confident pushing back against his job cuts, encouraged by Trump's reminder in early March that staffing decisions rested with department secretaries, not with Musk.

READ MORE: Musk, facing criticism and falling Tesla sales, to cut back DOGE work

Musk clashed with three of Trump's most senior cabinet members - Secretary of State Marco Rubio, Transportation Secretary Sean Duffy and Treasury Secretary Scott Bessent. He called Trump's trade adviser Peter Navarro a "moron" and "dumber than a sack of bricks." Navarro dismissed the insults, saying, "I've been called worse."

At the same time, Musk began to hint that his time in government would come to a close, while expressing frustration at times that he could not more aggressively cut spending.

In an April 22 Tesla conference call, he signaled he would be significantly scaling back his government work to focus on his businesses.

DOGE goes on

Musk's 130-day mandate as a special government employee in the Trump administration was set to expire around May 30. The administration has said DOGE's efforts to restructure and shrink the federal government will continue.

"The DOGE mission will only strengthen over time as it becomes a way of life throughout the government," Musk said.

Trump and DOGE have managed to cut nearly 12 percent, or 260,000, of the 2.3 million-strong federal civilian workforce largely through threats of firings, buyouts and early retirement offers, a Reuters review of agency departures found.

ALSO READ: US judge rules Musk's USAID cuts likely unconstitutional

Musk on Tuesday criticized the price tag of Republicans' tax and budget legislation making its way through Congress.

"I was disappointed to see the massive spending bill, frankly, which increases the budget deficit, not just decreases it, and undermines the work that the DOGE team is doing," Musk told CBS News.

His political activities have drawn protests and some investors have called for him to leave his work as Trump's adviser and more closely manage Tesla, which has seen falls in sales and its stock price.

Musk, the world's richest person, has defended his role as an unelected official who was granted unprecedented authority by Trump to dismantle parts of the US government.

Having spent nearly $300 million to back Trump's presidential campaign and other Republicans last year, he said earlier this month he would substantially cut his political spending.

"I think I've done enough," Musk said at an economic forum in Qatar.