The rising popularity of the UK Independence Party has left it unable to defend itself against infiltration by members of the far-right British National Party, its leader Nigel Farage has admitted.
At least three Ukip candidates have appeared on a leaked BNP membership list, while another was expelled from the Conservative party for attempting to influence a planning decision for political motives.
Ukip has a record 1,739 candidates standing in next week's local council elections, but its surging support has left it a victim of its own success, with its party machine struggling to cope.
The leader of the UK Independence Party is facing embarrassing questions after it emerged that Chris Scotton, 24, who was Ukip's Leicester candidate, has supported groups with extreme racist views.
Scotton 'liked' a series of controversial organisations including the English Defence League (EDL) and a site suggesting racism is 'just ethnic banter'.
Ukip were forced to suspend Mr Scotton after his apparent links to the EDL were published by MailOnline.
Referring to possible BNP sympathisers in his ranks, Farage this week appeared resigned to the fact that "one or two will have slipped through the net", saying that he "didn't have the party apparatus to fully vet 1,700 people".
Alan Ryall, a Ukip candidate in Wickham, Suffolk, admitted he had been a BNP member for "one or two years", but now found it "too extreme".
Ryall's name is included on a list of BNP candidates that was leaked in 2009.
Prospective Ukip candidates must sign a disclosure form confirming they have never been members of the BNP, and will do nothing to embarrass the party.
But Ryall, 63, said he had "nothing to hide", but had never been asked about his BNP links.
"I wasn't aware it was an issue. I've never been asked the question," he told the Times. "They nearly had nobody in this area and they were going through the list of members and asked, could they put my name forward. I said fine."
Peter Lucas, a Ukip town councillor in Ferndown, Dorset, was listed in the leaked BNP document as a student member.
But yesterday Lucas denied ever signing up to the BNP, saying the list included the names of people who had merely made inquiries. He claimed he had only got in touch with the party to find out more about its policies.
Chris Byrne, a prominent Ukip member who is standing in Surrey, also appears on the BNP membership list. But he also denied any links to the BNP. "That wasn't me. Someone's obviously put me on there," he said.
Byrne said he believed his name might have been entered into the BNP's membership maliciously, when his bank card went missing shortly after an argument with the BNP during a previous electoral contest.
Ukip yesterday promised to sack anyone found to have been a BNP member. However, the party has just four full-time paid campaign staff, compared with about 100 in each of the main parties.
Another candidate, Lister Wilson, received a four-month suspension during his time as Tory councillor for Cambridge County Council after attempting to use a planning decision to "settle scores" with a Lib Dem councillor.
An investigation into standards in 2011 found his behaviour "offensive" and "intimidating". Wilson denies any impropriety and said Ukip backed his "integrity".
Ukip recently hit a record 17% in national polls, and hopes to gain up to 50 seats.