Boris Johnson has become the first senior British politician to declare that Donald Trump's remarks about Muslims make him 'unfit' to serve as US President.
The Mayor of London stepped up his attack on the Republican Presidential contender, declaring that his "stupefying ignorance" proved he was "clearly out of his mind".
Mr Johnson hit out after Trump claimed that there were "no go areas" in London where police feared for their lives because of a threat posed by Muslims.
The property tycoon and reality TV star had sparked a backlash after calling earlier this week for a blanket ban on all Muslims from entering the United States, to reduce the terror threat.
David Cameron has already condemned Mr Trump's remarks, but the Mayor of London has now gone much further, claiming that they are a bar to him holding high political office.
"I think Donald Trump is clearly out of his mind if he thinks that's a sensible way to proceed, to ban people going to the United States in that way, or to any country," Boris said on Wednesday.
"What he's doing is playing the game of the terrorists and those who seek to divide us. That's exactly the kind of reaction they hope to produce.
"When Donald Trump says there are parts of London that are ‘no go’ areas, I think he’s betraying a quite stupefying ignorance that makes him frankly unfit to hold the office of President of the United States.
“I would invite him to come and see the whole of London and take him round the city - except I wouldn’t want to expose any Londoners to any unnecessary risk of meeting Donald Trump."
Trump had said this week: "We have places in London and other places that are so radicalised that the police are afraid for their own lives. We have to be very smart and very vigilant."
An online petition demanding Trump be barred from visiting the UK on grounds of his 'hate speech' reached more than 100,000 signatures on Wednesday morning - guaranteeing a debate in the Houses of Parliament.
But the Mayor joined George Osborne - who called for Trump to be debated rather than banned - in saying that it was the wrong move to stop Trump from entering the country.
“It’s a free country and you can’t stop people, unless he’s guilty of a criminal offence.”
Downing Street today made plain it felt Trump was trying to rally his base, with the Prime Minister's spokesman saying: "The commentary that was made by Mr Trump was clearly playing to an American audience, ahead of the primaries in six weeks' time."
Boris was born in New York and holds an American passport, which in theory makes him eligible to run for US President himself.
The Mayor yesterday nailed how many Londoners felt, using his first response to Trump to say he wouldn't want to visit some areas of New York in case he met the billionaire.
Yet his new broadside at the Republican hopeful makes clear relations between parts of the Conservative party and the Trump campaign are at an all-time low.