Keir Starmer congratulated Donald Trump on his 'historic victory' today after he swept swing states.
The Republican is set for an extraordinary return to the presidency, with the world braced for his bullish approach.
And Sir Keir is facing a particular challenge after tensions flared over Labour supporters helping to campaign for the Democrat contender.
The PM has also repeatedly clashed with billionaire Elon Musk, who looks certain to end up playing a key role in the Trump administration.
Sir Keir said in a statement this morning: 'Congratulations President-elect Trump on your historic election victory. I look forward to working with you in the years ahead.
'As the closest of allies, we stand shoulder to shoulder in defence of our shared values of freedom, democracy and enterprise.
'From growth and security to innovation and tech, I know that the UK-US special relationship will continue to prosper on both sides of the Atlantic for years to come.'
Foreign Secretary David Lammy - who once branded Mr Trump a 'neo-Nazi-sympathising sociopath' - said: 'The UK has no greater friend than the US, with the special relationship being cherished on both sides of the Atlantic for more than 80 years. We look forward to working with you and @JDVance in the years ahead.'
Sir Keir ordered Cabinet ministers to stay silent on the election for fear of further inflaming the situation. But MPs have been unable to resist expressing their dismay on social media this morning.
Meanwhile, jubilant Reform MPs have been demanding that Sir Keir uses Nigel Farage's close friendship with Mr Trump to prop up the Special Relationship.
Keir Starmer is braced for the fallout from a possible Donald Trump presidency today as Labour MPs quietly hope for a Kamala Harris win
The PM has also repeatedly clashed with billionaire Elon Musk , who could end up playing a key role in a Trump (pictured) administration should the Republic emerge victorious
Sir Keir said in a statement this morning that he 'looks forward to working with' Mr Trump 'in the years ahead'
Nigel Farage said the UK must 'roll out the red carpet' for Mr Trump
Labour MPs took to social media to respond to the Trump victory in the US election
Tory frontbencher Andrew Griffith mocked Labour for sending activists to help the Harris bid
MPs have been unable to resist expressing their dismay at Kamala Harris's (pictured) defeat on social media this morning
Government sources have been playing up a private dinner Sir Keir and Foreign Secretary David Lammy had with Mr Trump in New York in September, insisting they had struck up a good relationship.
But during the bitter campaign Mr Trump's team filed an official complaint with federal election authorities accusing Labour of making 'illegal foreign national contributions'.
Around 100 activists travelled to swing states to canvass for the Harris ticket - although Labour insisted they went at their own expense.
There was also anger that senior figures from the party went to the Democratic National Convention in August and met Harris aides. The two parties have long-term ties.
In interviews at the Commonwealth summit in Samoa last month, Sir Keir tried to gloss over fears the spat could derail the Special Relationship if Mr Trump wins.
He insisted that the situation needed to be 'put in perspective' as volunteers from parties have gone to help out in 'every previous election'.
As a backbench MP in 2018, Mr Lammy wrote in Time magazine: 'Trump is not only a woman-hating, neo-Nazi-sympathising sociopath.
'He is also a profound threat to the international order that has been the foundation of Western progress for so long.'
And in a 2018 Times article ahead of Trump's first visit to the UK, Lammy committed to be one of 'tens of thousands on the streets, protesting against our government's capitulation to this tyrant in a toupee.'
In a round of interviews yesterday, Health Secretary Wes Streeting was asked for his views on the election.
Tory MPs ridiculed Mr Lammy for his abrupt shift on Mr Trump
He joked 'I've got lots of thoughts but not ones I'm going to share as a government minister.'
Mr Streeting was challenged about a 2017 social media post where he described Trump as an 'odious, sad, little man', adding: 'Imagine being proud to have that as your President.'
He told Good Morning Britain: 'The Prime Minister and the Foreign Secretary have been working hard to build a relationship with President Trump and his team, so that in the event that he is elected as the next president of the United States, we start with the strong working relationship which is in our national interest and in the interests of the United States as well.
'And it's not often I would pray and aid Nigel Farage, the leader of the Reform Party but, as he said overnight, President Trump, you know, he speaks as he finds.
'He had a very good meeting with Keir Starmer not too long ago, and of course, he'll be aware of things that we've we've all said in the past…'
Mr Streeting said: 'The important thing about this relationship, this transatlantic relationship, is one of the most important alliances that Britain has.
Jubilant Reform MPs have been demanding that Sir Keir uses Nigel Farage's close friendship with Mr Trump to prop up the Special Relationship.
Some Labour MPs were unable to resist reposting pro-Democrat messages during the campaign - despite Sir Keir urging people to keep their counsel
'We have shared values and shared strategic interests, and with war on our continent, in Ukraine, war in the Middle East and wider geopolitical threats, it is in the UK and United States' national interests to work closely together.
'That won't change, whoever the occupant of the Oval Office is, and we only have hours now to wait until polls close.'
Senior government sources have poured cold water on Mr Farage's suggestion that he could act as a 'conduit' between his old ally Mr Trump and the UK government.
'That's good of him,' one said witheringly.