Joe Biden has overtaken Donald Trump by the smallest of margins in two key battleground states, as the race for the White House goes down to the wire.
The Democrat candidate jumped into first place in Republican-held Georgia and Pennsylvania for the first time, meaning if he can stay out in front he will clinch the US presidency.
Counting is still underway, with the results due to be updated throughout the day by election administrators.
US election 2020 live: Follow the latest updates
Currently, Mr Biden is ahead by 1,561 votes in Georgia and 12,390 in Pennsylvania - razor-thin considering around 10 million votes have been cast in the two states.
Sky News understands Mr Biden's campaign are "thrilled" at the developments, with campaign staff said to be "elated" and "confident".
But Mr Trump's team are insisting "this election is not over".
They vowed to challenge the Georgia result because they are "confident we will find ballots improperly harvested" - though cited no evidence - and claimed there are "many irregularities" in the Pennsylvania count.
Despite their attempts to delay a result, it looks like a nail-biting finish is in sight as the race to get to 270 Electoral College votes enters its final stretch.
Mr Biden is on 253 - and if he picks up 20 more with highly-prized Pennsylvania, he will win the White House and make Mr Trump the first one-term president since George HW Bush.
A result there was expected imminently, but an election official said it could be several days before the numbers are confirmed because of having to wait for ballots from members of the armed forces and voters living overseas.
Even without the Rust Belt jewel, Mr Biden would still get over the line with two of Nevada, Arizona, Georgia and North Carolina.
He is expected to address the nation tonight to make an appeal for unity and healing.
The most senior Democrat in Congress, Nancy Pelosi, is already treating victory as a given, saying it is "clear" Mr Biden will win and referring to him as "president-elect".
"His election is historic," she said - despite no more declarations having been made yet.
In Georgia, a recount will get underway due to the race being so tight.
Disputing Mr Trump's unsubstantiated claims of mass voter fraud, Gabriel Sterling, a local Republican official there overseeing the count, said "we're not seeing any widespread irregularities".
And Mitt Romney, the former Republican presidential candidate who ran in 2012, said it is "wrong" to claim the election was rigged - warning that doing so "damages the cause of freedom here" and "recklessly inflames destructive and dangerous passions".
Prime Minister Boris Johnson has tried not to wade into the row about the legitimacy of the election.
But he said he has "every confidence" in the checks and balances of the US system and promised to "work closely" with whoever wins.
Several records have already been broken this election - with Mr Biden, who is leading in the popular vote, getting the most votes achieved by any presidential candidate in history.
And Mr Trump has surpassed the total number of backers he had in 2016 by 3 million votes.
Analysis: Trump may lose, but eroded trust in democracy will last
By Sally Lockwood, news correspondent, in Pennsylvania
We could have known the result in Pennsylvania before now had postal votes been allowed to be counted before election day.
Instead, on-the-day ballots had to be calculated first under state electoral rules which revealed a lead for Mr Trump.
That margin has evaporated as mail-in ballots were tabulated.
Democrats were more likely to vote by post but the president has used his early lead as an opportunity to cry foul and fraud.
The Trump campaign's requests to discount postal votes by filing legal challenges in battleground states seem desperate attempts to delay the inevitable.
They already lost court rulings in three states yesterday.
A small number of ballots are being called into question and they would be unlikely to make any material difference to Joe Biden's current trajectory.
The more defining consequence of these legal disputes is an erosion of trust in America's democracy.
Mr Trump's repeated calls to "stop the count" may well have inspired an armed man to drive to Pennsylvania last night allegedly plotting to stop vote counters in the convention centre. The suspect now in police custody.
The Republican candidate's time in the White House may soon be over - but to the bitter end he is sewing dangerous distrust and division.
There will be no gracious concession speech, no unifying message, just alleging the election has been stolen from him - a parting gift to a country battered and bruised after a traumatic year.
