US attorney general William Barr, one of Donald Trump’s staunchest allies, is resigning amid lingering tension over the president’s baseless claims of election fraud and the investigation into Joe Biden’s son.

Mr Barr went to the White House on Monday, where Mr Trump said he submitted his letter of resignation.

“As per letter, Bill will be leaving just before Christmas to spend the holidays with his family,” Mr Trump tweeted.

The president has publicly expressed his anger about Mr Barr’s statement to the Associated Press earlier this month that the Justice Department had found no widespread election fraud that would change the outcome of the election.

Mr Trump has also been angry that the Justice Department did not publicly announce it was investigating Hunter Biden ahead of the election, despite department policy against such a pronouncement.

The president said deputy attorney general Jeff Rosen, whom he labelled “an outstanding person”, will become acting attorney general.

Mr Barr in his resignation letter said he updated Mr Trump on Monday on the department’s “review of voter fraud allegations in the 2020 election and how these allegations will continue to be pursued”. He added that his last day in the job will be December 23.

Despite Mr Trump’s obvious disdain for those who publicly disagree with him, Mr Barr had generally remained in the president’s good graces and has been one of the president’s most ardent allies.

Before the election, he had repeatedly raised the notion that mail-in voting could be especially vulnerable to fraud during the coronavirus pandemic as Americans feared going to polls.

But Mr Trump has a low tolerance for criticism from allies and often fires back in kind.

Mr Barr, who was serving in his second stint as attorney general, sought to paint himself as an independent leader who would not bow to political pressure, but Democrats have repeatedly accused him of acting more like the president’s personal attorney than the attorney general, and he had proved to be a largely reliable Trump ally and defender of presidential power.

Before releasing special counsel Robert Mueller’s full report on the Russia investigation last year, Mr Barr framed the results in a manner favourable to Mr Trump even though Mr Mueller pointedly said he could not exonerate the president of obstruction of justice.

He also appointed as special counsel the US attorney who is conducting a criminal investigation into the origins of the FBI’s probe into the 2016 election that morphed into Mr Mueller’s investigation of possible Trump-Russia co-operation, following the president’s repeated calls to “investigate the investigators”.

Mr Barr also ordered Justice Department prosecutors to review the handling of the federal investigation into Mr Trump’s former national security adviser Michael Flynn and then sought to dismiss the criminal charges against Mr Flynn, who had twice pleaded guilty to lying to the FBI. Mr Trump later pardoned him.

Mr Barr’s break from Mr Trump over election fraud was not the first. Earlier this year, he told ABC News that the president’s tweets about Justice Department cases “make it impossible for me to do my job”.