US Attorney General Jeff Sessions has apparently been sacked in what has been described as a "blatant attempt" by Donald Trump to end the investigation into Russian collusion.
Mr Sessions announced that he would be leaving just hours after the US president held a fiery White House news conference in the wake of a mixed midterm results.
The elections saw the Democrats seize the House of Representatives but lose further ground in the Senate.
In a letter to Mr Trump on Wednesday, Mr Sessions said: "At your request, I am submitting my resignation."
Almost immediately, Mr Trump announced on Twitter that Matthew Whitaker, who served as chief of staff to Mr Sessions, would become the acting attorney general.
Sky News' diplomatic editor Dominic Waghorn described Mr Whitaker as a "compliant replacement", who could have the power to end the inquiry being led by special counsel Robert Mueller.
In an interview with CNN last year, Mr Whitaker said: "I could see a scenario where Jeff Sessions is replaced with a recess appointment and that attorney general doesn't fire Bob Mueller, but he just reduces his budget to so low that his investigation grinds to almost a halt."
After an embattled relationship with the president – Sessions leaves his post saying he will always have the backs of the men and women in law enforcement pic.twitter.com/cUrIdPgPfN
— Emily Purser (@EmilyPurser) November 7, 2018
Mr Mueller has already brought charges against the former Trump campaign chairman Paul Manafort and other campaign figures, as well as against 25 Russians and three firms accused of meddling to help the president win in 2016.
The investigation is also looking into whether the president tried to obstruct the probe, as well as any financial misconduct by the Trump family.
Further progress was expected now that the midterms are over, with Mr Mueller having held back in recent weeks to avoid potentially influencing voters.
Senate Democrat leader Chuck Schumer said the president risks creating a "constitutional crisis" if he seeks to impede the Mueller investigation.
He said: "Given his previous comments advocating de-funding and imposing limitations on the Mueller investigation, Mr Whitaker should recuse himself from its oversight for the duration of his time as acting attorney general."
And Nancy Pelosi, who is set to become House Speaker, tweeted that it was "impossible" to read the firing as "anything other than another blatant attempt" by the president "to undermine and end" the Mueller probe.
The resignation – which some US media outlets are reporting as a firing – comes after more than a year of escalating tension between Mr Sessions and Mr Trump over the ongoing investigation.
Mr Trump has launched several personal attacks on Mr Sessions after the attorney general stepped aside from the inquiry, with the president blaming that decision for the appointment of Mr Mueller.
The president said a permanent replacement for him would be nominated "at a later date".
More from Jeff Sessions
During his news conference on Wednesday, Mr Trump lambasted the Russia probe as a "hoax" and repeated his insistence that there had been no collusion.
Mr Sessions – who was the first US senator to endorse Mr Trump – is the latest in a long line of White House officials to have left their post since he was elected, including press secretary Sean Spicer, environment chief Scott Pruitt and UN ambassador Nikki Haley.