Minor parties are preparing to exploit the public anger over MPs’ expenses when voters go to the polls in next month’s European elections.

Fringe groups have printed posters depicting mainstream politicians as pigs in troughs, and the issue is a constant theme at campaign launches.

Candidates have been briefed to tap into public anger over MPs’ expenses, with minor parties believing that it could help them to make significant gains on June 4.

Green Party candidates will depict themselves as the remedy to “Westminster’s snout-in-trough disease”, a spokesman told The Times.

The party will publish proposals this week for an overhaul of the expenses system. The plans will be distributed to voters and become a campaign tool for candidates.

The spokesman added that candidates would present themselves as an “antidote to the cynicism, apathy and anger” of the leading parties. He said that the Greens were “the only party presenting a positive vision” and hoped to gain votes as a result.

The issue of expenses was the standout theme at the British National Party’s campaign launch this week.

Nick Griffin, the party’s leader, said the furore was a huge advantage and claimed that public distrust in the mainstream parties could enable the BNP to win its first seat in the European Parliament.

He said that the public were sick of “fat, greedy politicians” and that BNP activists were being treated nationally like a “liberating army”.

The BNP’s poster campaign will continue the theme, with the slogan “Punish the Pigs”, depicting pigs wearing Tory and Labour rosettes.

BNP candidates and supporters, who had gathered in Grays, Essex, were entertained by a skit in which three men wearing pig masks played in troughs of money before they were chased away by “British workers” — men dressed in construction gear.

While other fringe parties believe that they can gain seats by highlighting the furore, the UK Independence Party said so much damage had been done to the main parties that it did not need to campaign on the issue.

“UKIP doesn’t feel the need to campaign on the expenses at Westminster story. There can’t be a single voter in the country that doesn’t already know what has been going on, given the way that the entire assembled media of the country have been discussing the story,” a spokesman said.

However, the party is itself vulnerable on the issue of politicians’ expenses, given that Ashley Mote, a UKIP MEP, was imprisoned in 2007 for falsely claiming £65,000 in welfare benefits. Last month another UKIP MEP, Tom Wise, was charged with false accounting and money laundering over allegations that he misused nearly £40,000 in expenses.

Both men have been expelled from the party.

UKIP may still tap into the anti-politics mood among the electorate because it presents itself as a clear alternative to mainstream parties.

Among other minority parties is Libertas, a pan-European movement founded by Declan Ganley, a leading figure behind the successful campaign for a “no” vote in the Irish Republic’s referendum on the Lisbon treaty

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