Nigel Farage, the Ukip leader, will try to eat into Labour's heartland working-class vote in next month's European elections by travelling to Sheffield this week to deliver a strong anti-immigration message, with polls showing that his party has so far been undamaged by allegations of expenses misuse.

An ICM poll for the Telegraph found Ukip three points below Labour on 30% with the Tories trailing on 22% and the Liberal Democrats on 8%, very close to the threshold that would see Lib Dem MEPs wiped out in the European parliament.

One-third of Tory voters in 2010 say they will back Ukip in next month's European elections.

Farage's swing through Yorkshire this week is designed to be pitched at working-class Labour voters seen as the group from which he can make the greatest gains between now and the May poll.

He is likely to be dogged by further demands to make good on his suggestion last week that he was willing to have his European parliament allowances independently audited.

Conservative, Labour and Liberal Democrat MEPs all have their allowances annually audited to show they are being spent as taxpayers would expect. Farage has said the criticism is an establishment plot and that none of the money he has claimed has been used for personal gain.

Edward McMillan Scott, the Liberal Democrat MEP for Yorkshire and Humber, said it would probably take three hours of an accountant's time to look at the expenses.

Godfrey Bloom, who was one of the Ukip leader's closest political allies and shared a flat with him in Brussels, said it was "perfectly fair" for Farage to allow auditors to examine his European parliamentary funds.

Every MEP receives a general allowance of €4,299 a month for running offices and other constituency work in their own countries. They do not need to provide any receipts to justify their spending.

An opinion poll published on Sunday night showed that Ukip supporters do not approve of inequalities of wealth, suggesting some of Farage's current support could be eroded over the controversy.

The poll, conducted this month by ICM for the High Pay Commission, found that 80% of people thinking of voting for Ukip believe the gap in wealth between the rich and poor is important, and 46% very important – the same proportions as for the electorate as a whole.

Potential Ukip supporters thought inequality between rich and poor was more important than building homes, cutting welfare benefits and cutting taxes.

The poll showed 96% of Ukip supporters thought reducing immigration was important and 85% very important – more than the national average of 77% and 50% respectively. It found that 95% of potential Ukip supporters thought changing the UK relationship with the EU was important and 76% very important, compared with 68% and 37% of the general electorate.

Among Ukip supporters, 87% say UK pay gaps are unfair and often do not reflect how hard people work; 85% say current pay gaps make it harder for people on low pay to get by on what they earn; and 84% believe UK businesses are often run to make a fast profit, not benefitting workers or communities in the long term.

Farage found himself embroiled in a row with the French Front National leader, Marine Le Pen after he called her party antisemitic. Last week Farage rejected a suggestion from Le Pen that Ukip could join a coalition with the FN in the European parliament, citing "prejudice and antisemitism" in her party – although he also said Le Pen had "some good qualities" and was "achieving remarkable things".

Le Pen used an interview with the Sunday Times to hit back at the Ukip leader, saying Farage had made "defamatory" and "extremely disagreeable declarations" in an attempt to boost his popularity.