US President Donald Trump has named Mick Mulvaney, the director of the office of management and budget, as acting White House chief of staff.
Mr Mulvaney, 51, replaces General John Kelly, who is leaving by the end of the year.
In a tweet, Mr Trump described Mr Kelly as a "great patriot", who had served the country with distinction.
But earlier this year, General Kelly was forced to deny he had called Mr Trump an "idiot".
This came after the quote was included in a book about Mr Trump by the veteran investigative journalist Bob Woodward.
Gen Kelly is said to have used the description repeatedly and also allegedly said that "it's pointless to try to convince him [Donald Trump] of anything".
What about Trump's announcement?
The president said that Mr Mulvaney "has done an outstanding job while in the administration".
"I look forward to working with him in this new capacity as we continue to MAKE AMERICA GREAT AGAIN!" he tweeted.
I am pleased to announce that Mick Mulvaney, Director of the Office of Management & Budget, will be named Acting White House Chief of Staff, replacing General John Kelly, who has served our Country with distinction. Mick has done an outstanding job while in the Administration....— Donald J. Trump (@realDonaldTrump) December 14, 2018
In a tweet, Mr Mulvaney said the appointment was "a tremendous honour".
"I look forward to working with the president and the entire team. It's going to be a great 2019," he wrote.
Friday's announcement ends days of speculation about the new White House chief of staff - one of the most prestigious jobs in Washington.
While the president has the final say on policy, the chief of staff wields incredible informal power and influence, the BBC's Anthony Zurcher in Washington says.
Mr Trump's reported first choice for the post, Nick Ayers, made it clear last weekend that he did not want the post.
Who is Mick Mulvaney?
The former South Carolina state senator and Congressman first gained attention as a founder member of the staunchly conservative House Freedom Caucus voting bloc. In Congress he opposed the Affordable Care Act, known as Obamacare, and gun control.
He currently leads the Office of Management and Budget (OMB), which assists the president in meeting policy and budget objectives.
Last year, Mr Trump also named him as acting head of the Consumer Financial Protection Bureau which he ran simultaneously with the OMB until earlier this month.