Postal workers in Somerset are refusing to deliver election leaflets for the British National Party (BNP) alleging they are racist.
Unions say they will refuse to deliver the pamphlets
The Communication Workers' Union said on Tuesday members were following their consciences over the leaflets.
But the BNP said its democratic rights were being undermined.
A formal complaint would be lodged with the police, alleging criminal offences, over any undelivered mail said a spokesman for the party.
The Communication Workers' Union has produced its own brochure: Answering the Asylum Myth which it plans to distribute in Somerset.
Kevin Beazer of the union said: "We've got a national agreement between the union and the employer as regards the conscience clause.
"This means our postmen and women don't have to
deliver offensive material, and we find the BNP to be a fascist and racist party and therefore we don't have to deliver this material."
Arrested and charged
BNP officials said the party was planning legal action against the union.
A spokesman said: "We are going to issue a series of complaints to the police about union members interfering with the delivery of the mail, theft and criminal damage, if any of the election addresses are not delivered.
"We will make formal complaints of interfering with the mail and destroying the mail in the event of any individuals not receiving their elections material and ask for the individuals responsible for destroying the election material be arrested and charged."