Donald Trump's comments about Islamist radicalisation in London make him "unfit" to be US president, the city's mayor Boris Johnson has said.
The property tycoon and reality TV star, who is seeking the Republican nomination for next year's presidential election, has provoked widespread anger and ridicule after calling for a ban on Muslims entering the United States and claiming that parts of the British capital were "so radicalised" that police were "afraid for their own lives".
A petition calling for Mr Trump to be barred from entering Britain has received more than 200,000 signatures on the parliamentary website, meaning that it will be considered for debate in the House of Commons. At one point, the petition was lengthening by more than seven signatures a second.
But Chancellor George Osborne dismissed calls for Trump to be excluded from the UK, saying it was better for his "nonsense" views to be challenged in debate.
Downing Street has made clear that David Cameron regards the presidential hopeful's comments as "divisive, unhelpful and quite simply wrong".
But Mr Johnson is the first prominent British politician to suggest they render Mr Trump unfit for office.
The mayor told ITV News: "I think Donald Trump is clearly out of his mind if he thinks that's a sensible way to proceed. You can't ban people going to the US in that way, or indeed to any country.
"What he's doing is playing the game of the terrorists and those who seek to divide us. That is 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 is betraying a quite stupefying ignorance that makes him frankly unfit to hold the office of the president of the United States."
Mr Johnson joked: "I would invite him to come and see the whole of London and take him round the city, except that I wouldn't want to expose Londoners to any unnecessary risk of meeting Donald Trump."
Challenged in the House of Commons over calls for a ban on Mr Trump entering the country, Mr Osborne said that the outspoken Republican was "profoundly wrong" about Muslims.
But he added: "I think the best way to defeat nonsense like this is to engage in robust, democratic debate and make it very clear his views are not welcome."
Any issue on which a petition exceeds six-figure support is expected to be debated by MPs unless it is deemed "unsuitable" by the Commons petitions committee - or is being pursued "in another way".
The committee is not due to meet again until January 5.
Downing Street said any question of a ban on Mr Trump coming to Britain was "hypothetical" as it was not aware of any plans for him to visit.
"I think we would probably see Mr Trump's focus as being on the American presidential election," said a Number 10 spokesman. "I understand he has a primary to fight in six weeks and I'm sure that is what his focus will be on."
Scottish First Minister Nicola Sturgeon stripped Mr Trump of his membership of the GlobalScot business network, saying that he was no longer fit to act as a business ambassador for Scotland.
More than 20,000 people have now signed a petition calling for him to be stripped of his honorary degree from Robert Gordon University (RGU) in Aberdeen, where he was awarded a doctorate of business administration in October 2010.
Founder of the petition Suzanne Kelly said it was "one of many little acts of defiance" against Mr Trump that had since ballooned.
Users experienced difficulties accessing the page on Wednesday "which may have been caused by a high volume of signatures", a Government spokeswoman said.
Ms Kelly, 54, a contributing writer at Aberdeen Voice, said: "I never expected this in my wildest dreams. I am glad that so many people feel the same way that I do.
"The number of things that man said before was alarming. I think we had better start standing up to this kind of behaviour right now."
Asked about what impact the petition could have, Ms Kelly said: "There is a distinct possibility that he might get banned.
"Mr Osborne has had to change his mind on more than a few occasions in the past."
An alternative petition "Don't ban Trump from the United Kingdom" has since been set up on the parliamentary site by David Gladwin, with over 1,200 signatures.
Asked about suggestions she should ban Mr Trump from the UK, Home Secretary Theresa May said: "We don't comment on specific cases."
She went on to defend British police.
"On the question of policing in London, I can assure you that Mr Trump has got it absolutely wrong," said Mrs May.
"Police in London are not afraid to go out and police the streets. They do a fantastic job for us day in, day out.
"We have seen recently very good examples of how our police officers actually go forward in the line of duty when they are required to do so to protect our citizens here in London and elsewhere in the UK."
Referring to Mr Trump's remarks about banning Muslims, Mrs May said politicians should be "very careful".
She added: "As we are dealing with the issue of terrorism, actually what we need to do is to bring communities together, to be working to bring greater cohesion in communities, not seeking to divide them."
The number of signatories to the petition has now topped 325,000.