US President Donald Trump called London Mayor Sadiq Khan a "stone cold loser" minutes before his plane touched down on the runway on Monday ahead of a three-day state visit to the UK.
The president, who is due to meet the Queen and prime minister Theresa May during the trip, said Khan had done a "terrible job" and had been "foolishly 'nasty' to the visiting President of the United States", which he described as "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 tweeted.
Misspelling his name, he went on to compare Khan to New York City Mayor Bill de Blasio — in both competence and height — before noting that his plane was landing.
"Kahn [sic] 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!"
Trump arrived at Stansted airport in London just before 9am local time.
A spokesperson for Sadiq Khan said the comments were "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," they added.
Trump's attack was later picked up by de Blasio, who described himself as a "Sadiq Khan stan" and said he'd rather be compared to the London mayor than "Brexit Bojo", referring to former foreign secretary Boris Johnson.
The tweets followed an opinion piece by Khan published in the Observer on Sunday, in which he said it was "un-British to be rolling out the red carpet" for Trump's state visit.
Foreign secretary Jeremy Hunt, who is in the running to become leader of the Conservative party and replace Theresa May as prime minister, met Trump on the tarmac at Stansted airport. Asked by Sky News about the tweets, Hunt said that Khan "has made some pretty choice insults about Donald Trump" and that the "spat" started because he and other Labour politicians chose to boycott the visit.
However, in his Observer piece, Khan hinted at a string of Trump's actions that have sparked controversy in the UK throughout his presidency, including his decision to retweet anti-Muslim videos posted by the deputy leader of far-right group Britain First and his criticism of British authorities following the London Bridge terror attack in 2017.
At the time, the president accused Khan of having to "think fast" and come up with a "pathetic excuse" when the mayor told Londoners not to be alarmed in the wake of the attack — prompting Khan to call for the cancellation of his planned visit.
"Through his words and actions, he has given comfort to far-right political leaders, and it's no coincidence that his former campaign manager, Steve Bannon, has been touring the world, spreading hateful views and bolstering the far right wherever he goes," Khan wrote in Sunday's article, which he retweeted on Monday.
"That's why it's so un-British to be rolling out the red carpet for a state visit for a president whose behaviour flies in the face of the ideals America was founded upon — equality, liberty and religious freedom."
In an interview with the Sun newspaper last week, Trump backed Boris Johnson in the Conservative leadership race.
"I think Boris would do a very good job. I think he would be excellent," Trump said of the former foreign secretary, who officially launched his campaign on Monday morning.
This week's trip is Trump's first official state visit to the UK — a controversial invitation that will be met with significant protests. He all but avoided London during his last visit after tens of thousands of people pledged to turn out on the streets, including some who campaigned to fly a "Trump Baby" blimp over the capital.
Labour leader Jeremy Corbyn tweeted in support of Khan, who was a Labour MP before he became mayor, and the planned protests.
Miqdaad Versi, spokesperson for the Muslim Council of Britain, tweeted that Trump's attack on Khan "undermines any positioning of the US working with the UK" and highlights "how he is happy to be seen as a champion of one section of British society alone".
Trump will meet the Queen, the Prince of Wales, and the Duchess of Cornwall when he is officially welcomed to Buckingham Palace. He is not, however, scheduled to meet Meghan Markle, the Duchess of Sussex, who is on maternity leave following the birth of her son, Archie.
The duchess, who is originally from California, was a vocal critic of Trump during his presidential campaign, calling him "misogynistic" in a 2016 interview on The Nightly Show With Larry Wilmore and saying that she might remain in Canada, where she filmed Suits, if he won.
Asked by the Sun about her criticism, Trump said: “I didn’t know that. What can I say? I didn’t know that she was nasty.”
On Sunday, Trump falsely denied calling her "nasty" — a phrase he famously used to describe Hillary Clinton, and which has now become a badge of honour for her and her supporters. However, the interview with the Sun was recorded and published by the newspaper.
It is not his first offensive comment about a member of the royal family. In 2012, he defended the paparazzi after the French edition of Closer published photographs of Kate Middleton, the Duchess of Cambridge, sunbathing topless.