Jeremy Corbyn used a campaign visit to Stoke to accuse Ukip of "trading on prejudice" as the party's leader Paul Nuttall faced fresh questions about claims made on his website.
The Labour leader spoke to his party's activists ahead of Thursday's Stoke-on-Trent Central by-election, where Mr Nuttall hopes to snatch the seat.
But Mr Nuttall faced further claims about inaccuracies on his website after a training charity denied an assertion on his website that he had been on its board.
His website, which has been taken down for "scheduled maintenance" after it emerged that claims he lost "close personal friends" in the Hillsborough disaster were false, suggested that he was joining the board of the North West Training Council (NWTC).
A press release from September 2009 said the North West England MEP was invited to the post after accepting an invitation to visit the NWTC.
"They are doing a first-class job and I am thrilled at the honour of being a board member," Mr Nuttall said.
But the organisation's chief executive, Paul Musa, said Mr Nuttall had never served on the board.
He told the Guardian: "Mr Nuttall was never invited to become a board member of NWTC as this would need to be a directive of the NWTC board, who he never met."
A Ukip spokesman said he was "rather surprised that this has just cropped up" and insisted the press release was put out with NWTC's knowledge.
"If it had not been accurate, don't you think it would have been complained about at the time?" the spokesman said.
Victory for Mr Nuttall would be a severe blow for Mr Corbyn's leadership, as Labour has held the seat since its creation in 1950.
In an attack on Ukip's policies, Mr Corbyn said: "What do they offer to people who need a house? To children in school? To people in hospital? To those needing social care? To those wanting a secure job? To young people wanting an apprenticeship or the opportunity to go to college or university and make the most of their lives?
"Ukip offer nothing, nothing, nothing and nothing again to any one of those groups because all they can do is trade on prejudice, blaming minorities, dividing communities, always blaming somebody else.
"Behind it is the real Trojan horse of privatising our NHS, giving it away to the lowest bidder in order that we will all end up paying for healthcare. That is the real Ukip agenda."
He was speaking alongside his party's candidate Gareth Snell, who hopes to retain the seat for Labour following Tristram Hunt's decision to quit Parliament.
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