Donald Trump's claim that parts of London are "so radicalised the police are afraid for their own lives," has prompted a brilliant mocking response by Boris Johnson.
The London Mayor replied that Trump's "ill-informed" comment was "complete and utter nonsense", adding that crime was falling in both London and New York.
"The only reason I wouldn't go to parts of New York is the very real risk of meeting Donald Trump," Johnson said.
Trump was speaking on MSNBC on Tuesday morning just hours after he said keeping Muslims out would prevent the emergence in the US of no-go areas he claimed existed in Europe.
He said: "Look at what happened in Paris, the horrible carnage, and frankly, if you look at Paris, and I hate to do this because the Chamber of Commerce is going to go crazy, but Paris is no longer the same city it was.
"They have sections in Paris that are radicalised, where the police refuse to go there. They're petrified. The police refuse to go in there.
"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."
Cameron's spokesperson called Trump "divisive, unhelpful and quite simply wrong" for calling for a "total and complete shutdown of Muslims entering the United States".
Trump did not specify which parts of London he was referring to.
Johnson said: "As a city where more than 300 languages are spoken, London has a proud history of tolerance and diversity and to suggest there are areas where police officers cannot go because of radicalisation is simply ridiculous."
The Twitter reaction to Trump was predictably scathing.
I think I visited one of those places Trump referred to last night. Bought some excellent baklava and some fine Turkish coffee. #London— Paul Bernal (@PaulbernalUK) December 8, 2015
Parts of Donald Trump are so radicalised that people in London fear for their lives if he is elected— Sir Robin Bogg (@robinbogg) December 8, 2015
#TrumpOnCNN London doesn't have 'no-go' areas. You'd know if you knew what you were talking about. The world is over you Mr Trump. Time 2 go— Thomas Pegg (@TomPegg) December 8, 2015
Long-time Trump friend Piers Morgan, writing on Mail Online, said: "The best friends are the ones unafraid to look you in the eye and say: ‘You’re wrong.’
"I wasn’t afraid to do that on Celebrity Apprentice and I’m not afraid to do it again now."
He added: "Your demand for all Muslims to be banned from America is so completely, utterly and dangerously wrong that I can’t just sit back and say nothing."
Trump's comments are reminiscent of claims made on Fox News about supposed "no go areas" created by Muslim radicalisation in France after the Charlie Hebdo attacks in January.
But the claim was dismissed by experts, interviewed by The Huffington Post UK at the time.