Sadiq Khan has accused Donald Trump of behaving like a “12-year-old” as the spat between the Mayor of London and the US President continues.

The London mayor said that he was "disappointed" by the US leader's handling of the aftermath of the London Bridge terrorist attack in early June.

At the time, the US President tweeted criticism Mr Khan for, he claimed, saying there is “no reason to be alarmed”.

“At least 7 dead and 48 wounded in terror attack and Mayor of London says there is ‘no reason to be alarmed!”, he said.

In reality Mr Khan had said Londoners should not be alarmed by the increased police presence in the capital, rather than by the attack itself.

Asked by the New Yorker why he appeared to have “got under Trump’s skin”, the London Mayor replied: “The short answer is I don’t know, but I can understand if we were both aged twelve years old.”

Mr Khan said he was “disappointed” in Mr Trump’s comments and referenced a Nato policy that says members will come to each other’s defence to suggest that politicians should support each other.

“The basic rule amongst politicians and amongst leaders is a sense of solidarity—it is almost like our version of Nato Article 5,” he said. “So I was extremely surprised when the President of the USA, our strongest ally, said what he said. It was disappointing, to be honest.”

At the time, Mr Khan said he had “better and more important things to focus on” than Mr Trump’s tweets, while a spokesperson claimed the Republican had taken the Mayor’s words out of context.

That led the US President to tweet: “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!”

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Mr Khan later called for Mr Trump’s proposed state visit to the UK to be cancelled, saying he was “not sure it is appropriate for our government to roll out the red carpet”.

The pair had previously clashed over the US President’s plans to introduce a ban on immigration to the US from seven mainly-Muslim countries.

When the Republican first suggested the policy in 2015, Mr Khan called the billionaire businessman a “buffoon” and said: “His views are divisive, they are outrageous, and I hope he loses. Badly.”