Boris Johnson has said he has "every confidence in the checks and balances of the US constitution" as Donald Trump makes unsubstantiated claims of voter fraud in the country's election.
The US president has been an ally of Mr Johnson after backing him to become prime minister and supporting his position on Brexit.
Asked if he will miss working with Mr Trump if he loses the election, Mr Johnson told Sky News: "Lets be clear, the prime minister of the United Kingdom is always going to work very closely with whoever is president of the United States, and that's going to be the case whatever the outcome of this election."
Mr Johnson declined to comment on what a US administration led by former vice president Joe Biden would mean for Britain, and said: "If I were a voter in America I don't think I'd want anybody in another government commenting on my election.
"I think while the votes are being counted we should wait and see."
It has been claimed Mr Trump had given Mr Johnson his personal phone number and they spoke on a regular basis.
Mr Biden, who is leading the race to the White House, has expressed opposition to Brexit and claims that the Good Friday Agreement is threatened by the Internal Market Bill which is currently going through the House of Lords.
The Democratic candidate tweeted in September: "Any trade deal between the US and the UK must be contingent upon a respect for the Good Friday Agreement and preventing the return of a hard border. Period.
"We can't allow the Good Friday Agreement that brought peace to Northern Ireland to become a casualty of Brexit."
Downing Street said in a reply: "We will continue to work with our US partners to ensure our position is understood."
Barack Obama, who was president when Mr Biden was vice president, is said to have been offended after Mr Johnson previously claimed his part-Kenyan heritage made him anti-British.
Mr Biden has overtaken Trump by the smallest of margins in two key battleground states.
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.
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.
The first candidate to get to 270 Electoral College votes wins the US election.
Mr Biden is on 253 and will pick up 20 more if he wins the highly-prized state of Pennsylvania.
Mr Trump is on 214 and will be the first one-term president since George HW Bush if he loses.