The Church of England is to debate draft legislation banning clergy from supporting the British National Party.
The law would make it "unbecoming" or "inappropriate" for clergy to be members of parties whose race equality policies were at odds with the Church.
The ban was backed two years ago by the General Synod, the Church's national assembly.
Reverend Robert West, a BNP member from Lincolnshire, said he did not believe the ban would be legal.
The ban was proposed by a lay synod member who works for the police, Vasantha Gnanadoss.
In 2009 the Synod voted overwhelmingly in favour of the measures.
At the time she said a membership ban would send a clear message against racial prejudice to the public at large.
'Discrimination'
Rev West, the BNP's religious affairs spokesman, said: "I don't think the Church has got legal authority to do this and it would be in breach of the law.
"The Church cannot discriminate against citizens on the basis of their political affiliations if the party is legal. The Church cannot challenge the law of the land."
Rev West said of his own sermons: "I expand on the scriptures and sometimes they have something to say to politicians but I would not say vote for the BNP or don't vote for the Conservatives.
"I might say it out of the pulpit but I would make it clear people were free to disagree with me," he added.
The Church meeting, in London, will also be addressed by the Archbishop of York Dr John Sentamu and will debate new proposals for the setting of fees for weddings and funerals.
