The special relationship has stepped up a gear on social media after US president Donald Trump responded on Twitter to Theresa May's condemnation of his retweets of a UK far-right leader's anti-Muslim videos.
In his first attempt, the president tagged an account not associated with May before deleting it and re-sending it.
He wrote on Wednesday (29 November): "Theresa @theresamay, don't focus on me, focus on the destructive Radical Islamic Terrorism that is taking place within the United Kingdom. We are doing just fine!" However, the tagged account did not belong to May, but a woman named Theresa Scrivener.
The posts included unverified videos called "Muslim Destroys a Statue of Virgin Mary!" and "Muslim migrant beats up Dutch boy on crutches!"
But May's spokesperson said in a statement: "Britain First seeks to divide communities through their use of hateful narratives which peddle lies and stoke tensions."
However the plans for Trump's state visit to the UK remain in place.
Brendan Cox, the widower of MP Jo Cox, who was killed in June 2016 by Thomas Mair, 53, who shouted "Britain First" when he stabbed her, said in a tweet, saying: "Trump has legitimised the far-Right in his own country, now he's trying to do it in ours. Spreading hatred has consequences & the President should be ashamed of himself."
Other people took to social media to respond to Trump's use of "Twitter diplomacy" with America's closest ally.
Meanwhile, Britain First's Jayda Fransen has praised Trump for re-tweeting three anti-Muslim videos she originally posted and has urged him to intervene in her upcoming court case.
The 31-year-old far-right leader is currently on bail facing trial over four charges of causing religiously aggravated harassment, the Evening Standard reported.
In a video, Fransen said: "I am appealing to you for your help, I am appealing for your intervention before I am thrown in jail and other receive the same treatment for simply speaking out. God bless you Donald Trump," Fransen said.
Fransen was convicted of religiously aggravated harassment in 2016 after shouting at a woman wearing a hijab during a "Christian patrol" of a predominantly Muslim neighbourhood. She was fined £2,000 at Luton and South Bedfordshire Magistrates' Court.