Ukip is the party for "decent supporters of the BNP", its deputy chairman Neil Hamilton has said.
The former Tory MP told the BBC that there were "a lot of decent people" who had previously voted for the BNP, because they felt their concerns about immigration were not being heard.
"They feel their communities are being swamped by immigrants from outside, whether they are from Eastern Europe or from other parts of the world.
"Now those people, the decent supporters of the BNP, from the last election, who weren't true BNP supporters at all, I am sure that quite a few of them are voting for a respectable alternative, which is Ukip."
The party unveiled the slogan "Love Britain. Vote Ukip" at its Spring conference, which commentators were quick to note the similarity to the BNP's former tagline "Love Britain. Vote BNP."
Leader Nigel Farage said it was "our slogan now".
Nick Griffin, the BNP leader which is expecting a dismal showing at next month's European elections, said Ukip are stealing the slogans of his party.
"They [Ukip] are using all of our rhetoric, they are using our slogans, they are recycling our posters and people like it," he told the BBC's Pienaar's Politics.
"The only difference is that Ukip won't deliver."
The BNP's policy was to "shut the door - we don't want anyone, black brown, green or white" not merely reduce immigration, Griffin said.
Farage has previously said he is "quite proud" that he has successfully persuaded BNP voters to switch their support to Ukip.
"What we did, starting with the Oldham by-election in the north of England, is for the first time try and deal with the BNP question by going out and saying to BNP voters; 'if you're voting BNP because you're frustrated, upset, with the changes in your community but you're doing it holding your nose because you don't agree with their racist agenda, come and vote for us,'" he said.
"I would think we have probably taken a third of the BNP vote directly from them, I don't think anyone has done more, apart from Nick Griffin on Question Time, to damage the BNP than Ukip and I am quite proud of that."