• The president said the federal agency overseeing the handover must "do what needs to be done", even as he vowed to keep contesting his election defeat.
• The General Services Administration (GSA) said it was acknowledging Mr Biden as the "apparent winner".
Donald Trump has accepted a formal US transition should begin for President-elect Joe Biden to take office.
The president said the federal agency overseeing the handover must "do what needs to be done", even as he vowed to keep contesting his election defeat.
The General Services Administration (GSA) said it was acknowledging Mr Biden as the "apparent winner".
It came as Mr Biden's victory in the state of Michigan was officially certified, a major blow to Mr Trump.
After the GSA announcement, the Pentagon said it would provide "support to the Biden team... in a professional, orderly, and efficient manner that is befitting of the public's expectation of the Department and our commitment to national security".
How did the Biden team respond?
It welcomed the start of the transition process as the Democratic president-elect gears up to be sworn in on 20 January.
What does GSA being allowed to preliminarily work with the Dems have to do with continuing to pursue our various cases on what will go down as the most corrupt election in American political history? We are moving full speed ahead. Will never concede to fake ballots & “Dominion”.— Donald J. Trump (@realDonaldTrump) November 24, 2020
"Today's decision is a needed step to begin tackling the challenges facing our nation, including getting the pandemic under control and our economy back on track," a team statement said.
"This final decision is a definitive administrative action to formally begin the transition process with federal agencies."
Earlier, Mr Biden unveiled a foreign policy and national security team consisting of old colleagues from his years in the Obama administration.
He will appoint Anthony Blinken as secretary of state and John Kerry as climate envoy, while Janet Yellen is tipped to be the first female US treasury secretary.
What did Trump say?
Mr Trump tweeted as the GSA, which is tasked with formally beginning presidential transitions, informed the Biden camp that it would start the process.
Administrator Emily Murphy said she was making $6.3m (£4.7m) in funds available to the president-elect.
...fight, and I believe we will prevail! Nevertheless, in the best interest of our Country, I am recommending that Emily and her team do what needs to be done with regard to initial protocols, and have told my team to do the same.— Donald J. Trump (@realDonaldTrump) November 23, 2020
While pledging to keep up the "good fight", the president said: "Nevertheless, in the best interest of our Country, I am recommending that Emily and her team do what needs to be done with regard to initial protocols, and have told my team to do the same."
Ms Murphy, a Trump appointee, cited "recent developments involving legal challenges and certifications of election results" in her decision to send the letter.
She said she had received no pressure from the White House over the timing of her decision.
"To be clear I did not receive any direction to delay my determination," said Ms Murphy's letter to Mr Biden.
"I did, however, receive threats online, by phone, and by mail directed at my safety, my family, my staff, and even my pets in an effort to coerce me into making this determination prematurely.
"Even in the face of thousands of threats I have remained committed to upholding the law."
She had faced criticism from both political sides for failing to begin the transition process sooner, usually a routine step between the election and the inauguration.
Ms Murphy missed a deadline on Monday set by Democrats in the House of Representatives to brief lawmakers about the delay.