The former legal adviser to the BNP has said the party includes "hardcore Nazis".
Lee Barnes left the far-right party earlier this month and then wrote on his blog that he was thinking of starting up a new party.
He said the BNP was "comprised on one the hand of hard-core Nazis on one wing and, on the other, civic nationalists. Both sides despise each other.
"The Nazi wing are in complete denial about the party's recent constitution change to allow ethnic minority members in, whilst the moderate wing are up in arms that the party did not do this sooner.
"Such internal ideological tensions cannot be resolved. Therefore, it is better that a split occurs now.
"Let the Nazis keep the BNP and return it to its National Socialist roots.
"There will always be a party required for Nazis, and though the NF [National Front] seeks to fill this gap, the BNP Nazis are a far more intelligent sort than the NF ones."
Mark Gardner, of the Community Security Trust, said: "Everybody knows that the BNP has tried desperately hard to shed its neo-Nazi image in recent years. The only question has ever been whether the changes are real or fake. In that regard, Lee Barnes's claims certainly make for interesting reading."
A spokesman for the BNP said: "Anyone who has any sympathy for Nazism will get thrown out of the party. We do not tolerate it. Many members' parents fought against the Nazis. If we took action for libel against every single lie we'd end up with 100 cases a week."