Brexit Party leader Nigel Farage says the Labour Party is “vulnerable in the most extraordinary way” in Leave-voting areas in the north of England.
Mr Farage was speaking at a rally in Pontefract, West Yorkshire, part of Labour MP Yvette Cooper’s constituency.
“The passion seems even stronger in Labour Leave areas than in Conservative Leave areas,” he told the audience.
But Ms Cooper said Mr Farage’s no-deal Brexit stance was a threat to manufacturing jobs.
Elections for 73 MEPs to the European Parliament will take place on 23 May.
Mr Farage told the rally: “It’s areas like this where I think the Labour Party is vulnerable in the most extraordinary way.
“This is a 70% Leave constituency, these five towns voted Leave by a massive margin.
“You’ve got a member of Parliament who, at the general election a year later, promised to honour the result, and has spent the last two years, effectively, trying to stop Brexit from happening.”
He said there was “real anger” in these places.
Mr Farage, who was UKIP leader during the 2016 EU referendum, told the rally that the European election campaign was the “biggest and most important campaign that I’ve ever fought”.
“Not only can we win this, we can transform the political landscape of this country.”
He said the party would “put up a full slate of policies” after the elections – including the scrapping of the HS2 rail project – but would not have a manifesto for the European elections.
Labour’s Ms Cooper said there was a “real responsibility” on “all political parties to come together” and “try and get a workable” Brexit deal and counteract Mr Farage’s message.
Former Conservative MEP Daniel Hannan said there was an alternative Brexit to the one being argued for by Mr Farage, but it would take a different prime minister to secure it.
“There must be space for people who want Brexit to happen in a liberal, agreed, orderly and grown-up way,” he told BBC Politics Live.
“Not every Leaver is for the hardest kind of aggressive future relationship with the EU. Most of the people who voted Leave want us to have good and cordial relations with our European neighbours.”
Cross-party talks between the government and Labour are continuing to try to find a way through the impasse.
The UK had been due to leave the EU on 29 March, but the deadline was pushed back to 31 October after Parliament was unable to agree a way forward.