The BNP said the list was published maliciously
A list showing the full contact details of British National Party activists has been published online.
The BNP says the list, which includes home phone numbers and e-mail addresses, dates from 2007 and some people are no longer members.
BNP deputy leader Simon Darby called it "an underhand political attack", saying an ex-employee was to blame.
Party leader Nick Griffin has complained to Dyfed Powys Police, who are investigating.
Mr Griffin said he was disappointed by the leak.
He said: "We'll be asking the police to investigate.
"Having spent a lot of money to secure our members' privacy we are disappointed that it's been breached.
"They know it's been in the hands of someone anti-BNP for a considerable length of time."
Mr Darby said the publication could lead to identity theft and endanger children named in the list.
He believes it was posted on Monday night by an ex-employee who had used it to send out party information to members.
Earlier this year the party obtained an injunction from the High Court in Manchester banning any publication of the list.
The anti-immigration party has won council seats in recent years, and took a London Assembly seat in May.