Ukip was embroiled in a fresh race row after it emerged that one of its local election candidates has described Islam as a "totalitarian ideology" that is " against everything modern Britain stands for".
Labour said the "deeply offensive" views - contained in an email apparently sent last year to the London council on which Heino Vockrodt wishes to sit - undermined Nigel Farage's insistence that he did not lead a racist party.
In an angry diatribe complaining about an alleged planning breach by "the Afghan community" in Brent, north London, Mr Vockrodt said parts of the borough resembled Afghanistan's Helmand province.
"Being Muslims, they ignored both refusals and did it anyway," he wrote of what he said were unlawful changes to a cultural centre.
Political correctness was preventing the council taking action, he alleged, " for fear of being called racists, when, just like in all the other cases where Muslims are grooming children to be sex slaves under the eyes of the authorities, the council does nothing".
He claimed the neighbourhood was "already exploding under the weight of segregation" and shopkeepers had been " squeezed out by Muslims".
"The entire parade - once lovely owner/occupier shops - resembles Helmand Province now. Time to call in MI5 and MI6 for sure," he added.
"Islam is a mono-cultural, totalitarian ideology. It is NOT a religion. It is against multi-culturalism and only promotes its own culture. It is against everything modern Britain stands for."
In the message, sent in the early hours of the morning to the planning enforcement department, he said that while he supported a multi-ethnic society, "t he only culture applicable to this country is British" and warned the council not to ignore the " winds of change".
It is a fresh embarrassment for Ukip as it tries to fend off racism allegations following a series of scandals involving candidates, and a controversial poster campaign warning about Europeans coming to take UK jobs.
Party leader Mr Farage has sought to dispel the image, accusing the media of stirring up hatred against Ukip and appearing on a platform alongside a number of the party's black and ethnic minority candidates.
"I don't care what you call us but from this moment on, please, do not ever call us a racist party. We are not a racist party," he declared.
But one of the party's prominent British Asian Ukip supporters has quit, accusing Mr Farage of playing the race card to win votes.
Sanya-Jeet Thandi, a youth member who was previously put up to defend the party in television interviews, said its current direction was "terrifying".
The Labour leader of Brent council, Muhammed Butt, said: "Mr Vockrodt's odious comments are deeply offensive.
"Nigel Farage keeps claiming Ukip isn't a racist party, but that's a hard pill to swallow when its members have put forward an individual with such plainly bigoted views for election to public office.
"In Brent we draw strength from our diversity and the vibrancy it brings, and our community deserves better."
A Ukip spokesman said: "Ukip is a non-racist, non-sectarian party and all candidates and members are expected to uphold these values. Where evidence is produced to indicate a breach, it will be considered at the earliest opportunity by the national executive committee as part of an established disciplinary procedure."
Mr Farage expressed "surprise" at Ms Thandi's departure, suggesting she might have been "intimidated" into quitting.
"She may have been intimidated, I don't know quite why she changed her mind," he told BBC Radio 4's World at One.
"We hadn't changed in seven days, had we? Nothing had changed in seven days... She changed in seven days, we didn't.
"So it is surprising on that level.
"Really more surprising is one 21-year-old, who no longer held any position within the party at all - because since she had gone to the London School of Economics she had been coming under a fair bit of peer pressure over being in Ukip - one 21-year-old leaves and it is a big story for the BBC, it is huge on the front page of the national newspapers.
"I didn't see any coverage of the rally I had in London the week before, when we had four dozen of our black and ethnic minority candidates standing there saying we believe in Ukip."
Mr Farage insisted Ukip had the "best and fairest" immigration policy of any of the parties.
He also renewed his warning that 250,000 Romanians and Bulgarians could come to Britain over five years, despite official figures yesterday suggesting numbers had actually dropped 3,000 to 122,000 in the first three months after working restrictions were lifted.
"What you saw for the first few weeks was so small and statistically insignificant that just five people leaving, five people leaving could have produced those figures," he said.
"Any proper statistician will say that you have to look at these figures year on year."
He went on: "The floodgates have opened... Net migration in this country last year was up to 212,000.
"There are 2.2 million Romanians who have left already. They have gone to Spain, they have gone to Italy, they have gone to Germany.
"I have always said that most Romanians who will come to Britain will probably come here from Spain, given how dire the eurozone is.
"We can't get away from the fact that there are over 4,000 people a week from all parts of the European Union coming to settle in Britain."
He said he still believed 250,000 more Romanians and Bulgarians would come to Britain over five years.
"It could be far more than that, a lot depends on the economies. I suspect the biggest risk we have is the eurozone, which looks to be a catastrophe," Mr Farage added.