Video report by ITV News Political Correspondent Romilly Weeks
Donald Trump launched a trademark Twitter attack on London Mayor Sadiq Khan, calling him a "stone cold loser".
The US President sent the tweets as he touched down in the UK for a three-day state visit.
He 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……
He also mis-spelled the London Mayor’s name: “…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.”
A spokesman for Sadiq 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 and the First Lady, Melania, were greeted by Foreign Secretary Jeremy Hunt and US ambassador to the UK Woody Johnson at Stansted Airport on Monday morning for their three-day trip.
But the Foreign Secretary was quick to defend the president and told ITV News the Labour party should be "ashamed" at making "party political capital out of a state visit."
He continued: "They need to see the bigger picture here, this is a very, very important relationship in the world for the peace and prosperity of people across the world and that's what we are celebrating."
Mr Trump arrived in Britain having 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 and talked up the prospect of a Brexit trade deal.
He also took aim at Mr Khan, who on Sunday described the president as “just one of the most egregious examples of a growing global threat” and compared the language he has used to that of the “fascists of the 20th century”.
Asked before he left if he planned to meet Boris Johnson during the trip, he said: “Well, I think I may meet with him. He’s been a friend of mine. He’s been very nice. I have a very good relationship with him.”He went on: “I have a very good relationship with Nigel Farage, with many people over there (in the UK) and we’ll see what happens. I may meet with him. They want to meet. We’ll see what happens.”
Mr Trump also rebuked Mr Khan as the "twin" of New York's mayor, "except shorter", and denied that he had labelled the Duchess of Sussex as "nasty", again taking to Twitter to defend himself.
Ahead of the visit, he called on Britain to leave the European Union without a deal if Brussels refuses to meet its demands and urged the Government to send Mr Farage into the negotiations.
The controversy was further exacerbated on Sunday when the US ambassador to the UK, Woody Johnson, suggested the NHS would be on the table in post-Brexit trade talks.
Addressing the prospect of a transatlantic trade deal, Mr Trump said: “(We’re) going to the UK. I think it’ll be very important. It certainly will be very interesting. There’s a lot going on in the UK. And I’m sure it’s going to work out very well for them.
“As you know, they want to do trade with the United States, and I think there’s an opportunity for a very big trade deal at some point in the near future. And we’ll see how that works out.”
The Trumps will head to Buckingham Palace for a reception with the Queen, the Prince of Wales and the Duchess of Cornwall.
The Duke of Sussex will also attend a private lunch at the palace – although his wife, who is on maternity leave, will be at home with their four-week-old son, Archie.
In a potentially awkward start to the trip, Mr Trump has already had to deny making “bad” comments about Meghan.
He tweeted to say that he did not brand the American former actress “nasty” when he was recently confronted with comments she made before the 2016 US elections saying she would leave the country if he won.
He blamed the “Fake News Media” for the reports and, asked at the White House on Sunday evening if he was willing to apologise to the royal family, or to clarify the comments made to the Sun’s political editor, Mr Trump replied: “No, I made no bad comment. Thank you.”
A string of events have been prepared to honour the couple, including a ceremonial welcome in the Buckingham Palace garden before Mr Trump has the opportunity to inspect the Guard of Honour.
Elsewhere, royal gun salutes will fire in Green Park and at the Tower of London to mark the Trumps’ arrival.
They will then visit Westminster Abbey where the president is due to lay a wreath at the Grave of the Unknown Warrior.
Mr Trump and wife Melania are also due to have tea with Charles and Camilla at Clarence House.
The day is due to conclude with a state banquet at Buckingham Palace at which both the Queen and Mr Trump will give speeches.
Mr Trump’s arrival in the capital is due to be followed by a protest through central London against the visit on Tuesday, with the Trump Baby blimp made for his visit last year due to put in an appearance.