London Mayor Sadiq Khan’s office fired back after President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE called him a “stone cold loser,” saying that the U.S. leader represents a “growing far-right threat around the globe.”

"This is much more serious than childish insults which should be beneath the President of the United States,” Khan’s office said in a statement.

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“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,” Khan's office added.

Trump and Khan have often exchanged criticism of each other since Trump took office in 2017.

Khan wrote a fiery op-ed on Saturday ahead of Trump's visit.

Trump fired back Monday morning, tweeting that Khan has done a “terrible job” as mayor and has been “foolishly ‘nasty’ to the visiting President of the United States.”

The back-and-forth also comes after Trump on Sunday mocked Khan by comparing him to another frequent target of the president, New York City Mayor Bill de Blasio Bill de BlasioOVERNIGHT ENERGY: California seeks to sell only electric cars by 2035 | EPA threatens to close New York City office after Trump threats to 'anarchist' cities | House energy package sparks criticism from left and right EPA threatens to close New York City office after Trump threats to 'anarchist' cities New Year's Eve in Times Square to be largely virtual amid pandemic MORE (D).

Trump called Khan the “twin of de Blasio, except shorter.”

Trump kicked off his state visit Monday by meeting with Queen Elizabeth II and other members of the royal family. He is scheduled to meet with outgoing Prime Minister Theresa May Theresa Mary MayAre US-Japan relations on the rocks? Trump insulted UK's May, called Germany's Merkel 'stupid' in calls: report Bolton says Boris Johnson is 'playing Trump like a fiddle' MORE as well.

Trump's visit to the U.K. comes as the nation is divided in still undecided Brexit negotiations and searching for a prime minister. Trump has publicly supported the controversial conservative choice Boris Johnson, saying he would be "excellent."