The US President accused the capital’s mayor of giving a “pathetic excuse” for how he reassured Londoners after seven people were killed on London Bridge and in Borough Market.
Mr Trump had written: “Pathetic excuse by London Mayor Sadiq Khan who had to think fast on his “no reason to be alarmed” statement. MSM (mainstream media) is working hard to sell it!”
In response, a spokesman for Mr Khan said: “The mayor is focused on dealing with Saturday’s horrific and cowardly attack and working with the police, the emergency services and the Government to keep London safe.”
During a news conference on Monday, the mayor even said that some state visits were “welcome, some less so” – a possible reference to Mr Trump’s expected trip to the UK.
After several repeated questions from journalists, Prime Minister Theresa May suggested at a campaign event that Trump had been wrong to criticise the mayor.
Mr Khan’s former Labour colleague in the Commons, David Lammy, had a less subtle response to the President.
Mr Lammy wrote: “You are truly beneath contempt. You are just a troll. Show some bottle please PM. Cancel the state visit and tell Trump where to get off.
“You demean your office by misquoting and smearing the Mayor of a city that has just been attacked and is also the capital of your close ally.
“You besmirch the presidency, you taint previous Presidents with your behaviour & you bring shame on your great country and its great people.”
The war of words had began after the London mayor gave an interview on Sunday morning in which he urged people not to be frightened by an increased police presence.
He had said: “My message to Londoners and visitors to our great city is to be calm and vigilant today. You will see an increased police presence today, including armed officers and uniformed officers.
“There is no reason to be alarmed by this. We are the safest global city in the world.
“You saw last night as a consequence of our planning, our preparation, the rehearsals that take place, the swift response from the emergency services tackling the terrorists and also helping the injured.”
This had been followed by Mr Trump firing off an incendiary barrage of tweets attacking Mr Khan for his plea for calm – and promoting his contentious travel ban on several Muslim-majority countries.
Mr Khan’s spokesperson had described the first round of presidential tweets as “ill-informed” – and said they had deliberately taken his remarks out of context.
On Monday afternoon, the White House insisted Mr Trump was not picking a fight with the mayor – but just raising an issue of national security.
Tensions between Westminster and the White House have been fraught for several weeks after intelligence gathered by the UK security services on the Manchester Arena attack was leaked to US media.