Theresa May hit back at Donald Trump for criticising Sadiq Khan
But the Prime Minister rejected calls to call off the US President's state visit to Britain currently planned for later this year.New York-born Foreign Secretary Boris Johnson also backed the visit to go ahead.
The Conservative politicians spoke after keen Tweeter Mr Trump fired off messages deriding Mr Khan's handling of the London Bridge terror attack.
The President – who in 2015 claimed extremist Islamism made parts of London no-go areas for police – mocked the Muslim mayor's assurance to Londoners seeing armed police that there was "no reason to be alarmed".
In the aftermath of the London Bridge attack, we have been working with Sadiq Khan
Mr Trump said that was a nonsensical statement given that terrorists had just killed and wounded dozens.
Brushing aside the hostile response, he then Tweeted that Mr Khan had made a "pathetic excuse" for his original comment.
Mrs May said that although the Anglo-US bond was "our deepest and most important defence and security relationship … I think Donald Trump is wrong in the things he said about Sadiq Khan.
Mr Trump said it was a nonsensical statement by Sadiq Khan
"In the aftermath of the London Bridge attack, we have been working with Sadiq Khan. Party politics is put to one side, we work together."
Praising the Labour mayor, she noted that he had been part of the Government's emergency Cobra committee efforts to design the right to help ensure the right response to Saturday's attack and "get London moving again".
However, asked if the President's visit to Britain will go ahead, she replied: "Yes."
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Earlier, former London Mayor Mr Johnson said there was "no reason" to cancel the state visit, which Mrs May extended to Mr Trump in Washington in January on the Queen's behalf.
Boris Johnson said there was "no reason" to cancel the state visit
Mr Johnson said: "But as far as what Sadiq Khan has said, about the reassurances that he has offered the people of London, I think he was right to speak in that way…
"He is entirely right to say what he said to reassure people of his city about the presence of armed officers on the streets."
Mr Khan himself was scathing about Mr Trump, telling ITV1's Good Morning Britain: "We're not kids in a playground, he's the President of the US. I'm too busy to respond to his tweets, isn't he busy?"
The mayor, who last year slammed Mr Trump's "ignorant" calls for a ban on Muslim entry into the US, added: "It takes two to tango. I'm not tangoing with this guy, I've got better things to do."
Sadiq Khan leads sombre vigil for London terror victims Tue, June 6, 2017
London Mayor Sadiq Khan lead a vigil to commemorate the victims of the London Bridge terror attack. Hundreds attended the event outside London's City Hall
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London Mayor Sadiq Khan bows his head
He had opposed a "premature" Trump state visit after the president restricted refugee entry, on grounds that such visits were granted "to leaders of countries who are respected" and had a good track record but he added: "Let's not overdo it, I'm not in a war with Donald Trump."
In America, Mr Trump's oldest son sprang to his father's defence, saying Mr Khan must "do something to fix the problem rather than sit there and pretend there isn't one".
Donald Trump Junior told Good Morning America: "Every time he (the president) puts something out there, he gets criticised by the media all day, every day by everyone else, and then guess what? Two weeks later he's proven to be right – it happened again.
"We keep appeasing it, we keep saying 'OK, it's going to be great. We are going to hold fast, we are going to keep calm and carry on'.
Donald Trump Jr is running the president's real estate empire with his brother
"Maybe we have to keep calm and actually do something, I think that's what he is trying to say, because he's been proven right about it every time."
Mr Tump Jr – who with young brother Eric is running the president's real estate empire while their father is in office – added to ABC News: "I think it's time for the people there (in London) to act and do something a lot more proactive about what's going on today."
But Republican Senator Bob Corker told US media of Mr Trump's Tweets: "We live in a world today where unfortunately a lot of communication is taking place with 140 characters and probably it's best to refrain from communicating with 140 characters on topics that are so important."