Donald Trump has branded London mayor Sadiq Khan a "stone cold loser" in a tweet posted just moments before he touched down in the capital city for a three-day state visit.
The US president continued to say that Mr Khan should "focus on crime in London, not me".
"Sadiq Khan, who by all accounts has done a terrible job as Mayor of London, has been foolishly 'nasty' to the visiting President of the United States, by far the most important ally of the United Kingdom," he wrote.
"He is a stone cold loser who should focus on crime in London, not me...".
The tweet was posted around 11 minutes before he touched down at Stansted Airport, and followed comments made by Sadiq Khan openly opposing Mr Trump's state visit.
The mayor had previously said the UK should not "roll out the red carpet" for the president, and in an editorial, he called the US leader “one of the most egregious examples of a growing global threat”.
In a follow-up tweet, Mr Trump said that the London Mayor reminded him of New York City’s “very dumb and incompetent mayor”, Bill de Blasio.
He ended the tweets on a positive note: “In any event, I look forward to being a great friend to the United Kingdom, and am looking very much forward to my visit. Landing now!”
Responding to the tweets, a spokesman for the London mayor said that "childish insults" should be "beneath the president of the United States".
It’s not the first time the pair have traded barbs online.
In the aftermath of the London Bridge terrorist attack in June 2017, Mr Trump wrote: “At least 7 dead and 48 wounded in terror attack and Mayor of London says there is no reason to be alarmed!”
In the first few months of the Trump presidency Mr Khan called him “ignorant” in his views on Islam.
In return, Mr Trump challenged the London mayor to an IQ test.