Donald Trump has forced Jeff Sessions to resign as US Attorney-General in another destabilising move after losing the majority in the House of Representatives.

Mr Trump announced on Twitter that Matthew Whitaker, Mr Sessions’ chief of staff, will take over as interim Attorney-General until a permanent replacement is nominated.

In a letter overnight, Mr Sessions announced his plan to resign under pressure from Mr Trump, with the document beginning with: “At your request, I am submitting my resignation.”

The resignation was delivered to White House Chief of Staff John Kelly on Thursday morning (AEST), and came just a day after the midterm elections, which saw the Democrats regain control of the House, while the Republicans expanded their Senate majority.

We are pleased to announce that Matthew G. Whitaker, Chief of Staff to Attorney General Jeff Sessions at the Department of Justice, will become our new Acting Attorney General of the United States. He will serve our Country well….

— Donald J. Trump (@realDonaldTrump) November 7, 2018

Mr Sessions said goodbye to waving staff as he left is office for the last time Thursday, despite reportedly asking to remain until the end of the week.

The former attorney general’s request to stay a few more days was denied by Mr Kelly, CNN cited unnamed administration officials as saying.

“Kelly was very firm it had to be today,” an official was quoted as saying.

Justice Department employees, meanwhile, said they found out about Mr Sessions’ resignation via news reports.

The move was not entirely unexpected, as Mr Trump publicly and privately berated Mr Sessions after the Attorney-General recused himself from the investigation into possible collision between the Trump campaign and Russia last March.

Rod Rosenstein, Mr Sessions’ deputy, later appointed Special Counsel Robert Mueller to lead the probe.

“Sessions should have never recused himself, and if he was going to recuse himself, he should have told me before he took the job and I would have picked somebody else,” Mr Trump told the New York Times in July 2017.

Mr Trump continued his criticism throughout the past year, and in September, appeared to disassociate Mr Sessions from the administration completely, telling Hill TV in an interview: “I don’t have an attorney-general. It’s very sad.”

Shortly before Mr Trump announcement Mr Sessions’ resignation, the President held a chaotic press conference in which he was asked if was planning on making any changes to his team.

He did not mention Mr Sessions and said that he was overall “extremely” happy with his cabinet.

Mr Sessions’ departure immediately raises questions about the future of the Mueller investigation.

Democratic House of Representatives leader Nancy Pelosi on Thursday said the forced resignation was an attempt by Mr Trump to undermine the Mueller probe.

It is impossible to read Attorney General Sessions’ firing as anything other than another blatant attempt by @realDonaldTrump to undermine & end Special Counsel Mueller’s investigation.

— Nancy Pelosi (@SpeakerPelosi) November 7, 2018

She went on to call on Mr Whittaker to recuse himself from any involvement in the investigation. 

Mr Whitaker, who will now oversee the Mueller investigation, wrote an opinion piece for CNN in August, 2017, arguing that Mr Mueller would be going too far if he were investigating Mr Trump’s finances.

Incoming Democratic committee chairmen are expected to lead investigations into Mr Trump’s long-hidden tax returns, possible conflicts of interest from his business empire and any collusion with Russia in the 2016 election.

Ms Pelosi, who is expected to return as House speaker when the new Democratic majority takes over in January, said committee chairmen will decide how to proceed and make their recommendations to the Democratic caucus.

“But you can be sure of one thing: when we go down any of these paths, we’ll know what we’re doing and we’ll do it right,” the 78-year-old San Francisco liberal said.

On Capitol Hill, Senate Minority Leader Chuck Schumer said: “Protecting Mueller and his investigation is paramount. It would create a constitutional crisis if this were a prelude to ending or greatly limiting the Mueller investigation.”

Mr Schumer also said that Mr Whitaker should recuse himself given previous comments about defunding and putting limitations on the probe.

House Democrats accused Mr Trump of trying to “manipulate” the investigation during the lame-duck Congress.

-with AAP