Donald Trump has shared a racist tweet to attack Sadiq Khan following three killings in the city.
“London needs a new mayor,” Mr Trump wrote.
“Khan is a disaster – will only get worse!”
The US president made the comments while retweeting Katie Hopkins, a far-right, Islamophobic commentator.
Ms Hopkins highlighted some recent stabbings in London in the original tweet.
“This is Khan’s Londonistan,” she wrote.
Mr Khan is a British citizen, of Pakistani descent. The term ‘Londonistan’ is often used in a disparaging sense when referencing the British capital’s Muslim community.
The tweets are the latest in a long line of clashes between Mr Trump and Sadiq Khan, a practising British Muslim.
The mayor of London compared the US president to 20th-century fascists during Mr Trump’s recent state visit to the country.
The US president, in response, called the Labour Party politician a “stone cold loser”.
Mr Khan is currently grappling with knife crime in London.
A pair of teenagers were killed in different parts of London within minutes of each other on Friday afternoon, while three men were stabbed in a separate attack in the capital hours later.
On Saturday a man in his thirties died following a stabbing on a street in Tower Hamlets – the third fatality from violent attacks in the space of 24 hours.
At least 14 people have been arrested in connection with the attacks.
Experts previously told The Independent that Mr Trump’s comments could fuel harassment targeting Mr Khan, who is already under 24-hour police protection because of racist harassment and death threats.
Jacob Davey, the lead on far-right research at the Institute for Strategic Dialogue, said Mr Trump’s attacks on Mr Khan acted as a “nod and a wink” to his extremist supporters.
“There is this absolute obsession with Mr Khan amongst the alt and far-right, and it is intrinsically linked to the fact he’s a Muslim of Pakistani origin,” he said.
“It ties in with broader far-right conspiracy theories, including the idea of Islamisation and that white Europeans are being culturally and ethnically displaced globally.”
The feud between Mr Khan and Mr Trump dates back to 2016, when the London mayor called the president’s proposed Muslim immigration ban “divisive and outrageous”.