The British National party tonight vowed to take legal action after its entire membership list was published online in breach of a court injunction.
The party blamed disgruntled former employees for posting details of the names, postal and email addresses and ages of more than 10,000 supporters.
Details of the hobbies, professions and other areas of expertise of many members are thought to be included.
The membership list dates from late last year, the BNP said, and its publication had been prevented under a high court injunction obtained in April.
Embarrassed party bosses have complained to the police and are seeking legal advice to prevent the further spread of the data.
Simon Darby, the BNP's spokesman, said he found out this morning that the injunction had been broken, describing the posting as "malevolent and spiteful".
"This is being done to destabilise the party after a successful conference in Blackpool and before the elections for the European parliament in June next year."
He said the membership list, which was password protected and encrypted, had been stolen from the party. "This isn't a question of us mislaying the information, this is theft," he said.
Darby claimed the list contained the names of people who had never been members of the party as well as the names of current and former members. Although the party is taking action to take down the list, Darby conceded: "Once it's out, it's out."
"We are worried because kids' names are on the list. It is not information that should be in the public domain," he added. "We are always receiving death threats."
He said the party discovered the information had been leaked when its members started receiving unsolicited mail. "We found that members were being sent propaganda. We went to the Manchester high court and obtained an injunction. It cost tens of thousands of pounds," he said.
Darby said the party had complained to Dyfed Powys police – the BNP carries out much of its administration in Welshpool – but the force was unable to confirm that a complaint had been made.
The publication sparked alarm among the BNP's membership.
"I'm also on the list, what the fuck is going on? I could lose my job," posted one member on a north-west England BNP forum.
Another wrote: "God help anyone who is in the army, the prison service, health care, police officer or a teacher."
Since 2004, police officers have faced dismissal if found to be members of the BNP.