Editors of local newspapers in London boroughs targeted by the BNP have been called on to reject political advertising by the far-right party.
The National Union of Journalists (NUJ), Bectu, the media and entertainment union, and the campaign group, Expose the BNP, have written to local newspapers in Barking and Dagenham, Romford and Havering urging them to resist pressure from the BNP.
The party is fielding 154 candidates at the general election.
In 2008, Archant London, one of the country's biggest media groups, caused outrage after it printed BNP adverts for the Greater London Authority elections in its London newspapers, including the Ham & High and Hackney Gazette.
Jeremy Dear, general secretary of the NUJ, said: "Newspapers should not take money from an organisation that advocates racist policies that would directly discriminate against the communities they serve.
"Publishing ads from far-right organisations seriously threatens the reputation of a company's titles and journalists."
Alexandra Fawcett, a media lawyer at Mishcon de Reya, said: "Newspapers are entitled to publish or reject adverts as they see fit.
"In deciding whether to run the BNP's adverts, each paper will weigh the financial benefit of publishing against the likely reputational damage of associating their brand with an organisation like the BNP.
"As a private company, a paper's decision isn't open to challenge under the Human Rights Act even though refusing to run the adverts might impact on the BNP's right to free speech."
Mary Brodbin, of Expose the BNP, observed: "The BNP is not a normal, respectable or legitimate political party –- it is an organisation that aims to disenfranchise a significant section of British society, and it encourages violence to achieve those ends.
"No newspaper can afford to be associated with the BNP and its methods."
A spokesman at the Board of Deputies praised the campaign and said: "The BNP are entitled to peddle their twisted ideology just as newspaper editors and the NUJ are entitled to contribute to the fight against them by seeking to curtail their advertising campaigns.
"The NUJ is to be congratulated for its principled stance."
The Board is urging communities to "support any party of their choice which stands opposed to the destructive politics of hatred, to vote for freedom not fear, partnerships not prejudice, and hope not hate".
A spokesman for the BNP said that the appeal was "disgraceful" and an "outrageous" attempt to "silence free speech".