The British National Party is demanding a police investigation after a list of 10,000 members found its way onto the internet.
Nick Griffin, leader of the right wing party, is claiming that the leak, believed to have been by former members, puts supporters at risk of violence.
The list, which dates back to 2007, includes the names, addresses, jobs and phone numbers of more than 10,000 people, and even lists children whose parents are signed up to family memberships.
But, most worryingly, the list includes former and serving police officers (even though officers have been banned from being members since 2004), as well as teachers and members of the armed forces.
Griffin is describing the leak as "a disgraceful act of treachery" and has lodged a complaint with Dyfed-Powys Police claiming that the publication breached human rights and data protection laws.
He added to the BBC's Today programme that he had "no problem at all" about the professions of members being in the public domain, which was "a matter of public interest".
But actually revealing names and addresses was a "nasty piece of intimidation" and he went on to point his finger at the "Labour regime".
However, he added: "In terms of repositioning us as a party genuinely made up of ordinary British people from all walks of life that will actually do us good", he said.
In response to Griffin's accusations, Home Secretary Jacqui Smith said "everybody has a right to protection under data protection laws" but quipped that she was happy for the public to know she is a member of the Labour Party, asking: "I wonder why it is that BNP members are rather more ashamed of their membership".