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".
And Mr Biden's transition website was changed to a US government domain.
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.
"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.
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.
Did Republicans bring any pressure to bear?
Mr Trump's fellow Republicans have increasingly been breaking ranks over the transition, with several of them speaking out on Monday.
Tennessee Senator Lamar Alexander, who is retiring, released a statement saying that Mr Trump should "put the country first" and help Mr Biden succeed.
"When you are in public life, people remember the last thing you do," Mr Alexander said.
West Virginia Senator Shelley Moore Capito said "at some point, the 2020 election must end".
Senator Bill Cassidy of Louisiana noted the Trump legal team had failed to present evidence of the massive fraud which they claim stole the election, adding that the transition "should begin for the sake of the country".
Ohio Senator Rob Portman argued for Mr Biden to receive top-secret briefings on national security and to be updated by the Trump administration on plans for coronavirus vaccine distribution.
More than 160 business leaders had also urged Ms Murphy in an open letter to immediately recognise Mr Biden as president-elect.
"Withholding resources and vital information from an incoming administration puts the public and economic health and security of America at risk,″ they wrote.
What happened in Michigan?
One of two Republicans on the Michigan State Board of Canvassers joined the two Democrats to finalise the result. The other Republican abstained. Mr Biden won the state by more than 150,000 votes.
Abstaining Republican board member Norman Shinkle had suggested delaying the certification over irregularities affecting a few hundred votes in one county.
But his colleague, Republican Aaron Van Langeveld, on Monday said their duty was "simple" and there was no option but certification.
Mr Trump's legal team said they would still challenge Michigan's results.
Adviser Jenna Ellis said certification was "simply a procedural step". She added: "Americans must be assured that the final results are fair and legitimate."
But time is running out. On 14 December, Mr Biden's victory is set to be approved by the US Electoral College.
What about Trump's other legal challenges?
Mr Trump and his allies have suffered a string of court defeats in key states as they race to challenge the results.
His campaign has reportedly tried to convince Republican state lawmakers to appoint their own electors to vote for him instead of Mr Biden, but to no avail.
In Pennsylvania, a Republican judge on Saturday ruled that the Trump campaign had tried to "disenfranchise almost seven million voters" with no real evidence. The president's lawyers have appealed now to the Circuit Court of Appeals in Philadelphia.
The president's other legal efforts in the state have failed to change Mr Biden's lead of some 80,000 votes.
The Trump campaign has also called for another recount in Georgia, after an earlier recount by hand confirmed Mr Biden's win in the state.
In Wisconsin, a partial recount is under way by request of the Trump campaign. Election officials have accused Trump supporters of obstructing the state's recount of votes.
They said observers for Mr Trump were in some cases challenging every single ballot to deliberately slow down proceedings.