Trump tweeted that Khan, who has been highly critical of the state visit, “has done a terrible job as Mayor of London” and “has been foolishly 'nasty' to the visiting President of the United States”.
The President called himself “by far the most important ally of the United Kingdom”, saying Khan should “focus on crime in London” rather than on criticising Trump.
He then went on to insult Khan’s height, while spelling his name wrong.
.@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......— Donald J. Trump (@realDonaldTrump) June 3, 2019
The furious tweets came just minutes before the famous Air Force One presidential aeroplane touched down at Stansted Airport ahead of Trump's state visit, suggesting he had composed them in the air.
A spokesperson for the Mayor of London 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."
On Sunday, Khan wrote in the Observer newspaper that Trump has engaged in the “deliberate use of xenophobia [and] racism” as “an electoral tactic”.
Khan called it “un-British to be rolling out the red carpet this week for a formal state visit for a president whose divisive behaviour flies in the face of the ideals America was founded upon”.
He pointed to Trump’s comments that there were “very fine people on both sides” after a white supremacist rally in Charlottesville, Virginia ended with a counter-protestor’s death in 2017, comparing him to “the fascists of the 20th century”.
Trump and Khan have been locked in a feud since the President was a candidate in 2016, when the mayor called Trump's views on Islam “ignorant”. Khan’s intervention caused Trump to challenge Khan to an IQ test.
Last July Trump said Khan had “done a very bad job on terrorism”, having previously called the mayor “pathetic” over the 2017 London Bridge terror attack.
Thousands of people across the country are expected to protest against Trump’s visit on Tuesday when the President meets Prime Minister Theresa May in Downing Street.
It is thought that the infamous Trump baby blimp – a giant balloon showing the president as a nappy-clad baby – will reappear, and be joined by a giant robot of the President sitting on a gold toilet.