Donald Trump launched a Twitter tirade at Mayor of London Sadiq Khan as he arrived in the UK for his state visit.
The US president branded the London Mayor a "stone cold loser" in tweets sent out as Air Force One touched down at Stansted Airport on Monday morning.
Mis-spelling the London Mayor's name, Mr Trump wrote: "@SadiqKhan, 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 is a stone cold loser who should focus on crime in London, not me......
"...Kahn reminds me very much of our very dumb and incompetent Mayor of NYC, de Blasio, who has also done a terrible job - only half his height. 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."
In return the mayor's office fired back, saying Mr Trump was offering "childish insults".
A spokesman for Mr Khan said: "This is much more serious than childish insults which should be beneath the President of the United States.
"Sadiq is representing the progressive values of London and our country warning that Donald Trump is the most egregious example of a growing far-right threat around the globe, which is putting at risk the basic values that have defined our liberal democracies for more than 70 years."
Mr Trump's three-day trip will see him meeting the Queen and other senior royals, as well as Theresa May in her final days as Prime Minister.
Before arriving in Britain, Mr Trump had already created a considerable degree of political turbulence with comments on the Tory leadership race, Brexit and the Duchess of Sussex.
Speaking to reporters just before he left the US, he praised Boris Johnson and Nigel Farage, talked up the prospect of a Brexit trade deal, and rebuked Mr Khan.
The US President and Mr Khan have repeatedly clashed in recent years, including Mr Trump criticising the mayor's response to the 2017 London Bridge terror attack.
Mr Trump arrived on Air Force One at Stansted Airport, waving as he walked down the steps from the front of the jet, accompanied by the First Lady who wore a blue pencil skirt and blazer, dark sunglasses and heels.
An array of US military helicopters arrived around an hour ahead of the President's white and blue plane, which bears the US flag on the tail fin and the words United States of America.
There was a visible armed police presence around the airport and police vehicles parked alongside the runway.
Other passengers on Air Force One, including members of the media, disembarked from the rear of the plane while 20 troops from the RAF Regiment formed a guard of honour for Mr Trump.
Mr Trump and the First Lady made the short walk to helicopter Marine One which took them to central London.
A military officer saluted Marine One as it taxied off, followed by a second US helicopter and a UK police helicopter.
