A detailed membership list of the British National party containing names, addresses and telephone numbers was published on the internet this morning.
The list, which contains thousands of names, was published on Wikileaks, a website that purports to be a clearing house for information to be published anonymously.
A Guardian analysis of the data suggests the BNP had 11,560 members as of April this year, including one peer and several doctors and military personnel. The party appears to have benefited from a surge in female recruits, with one in eight party members now women.
The list reveals that the highest concentrations of members are in Leicestershire, Lancashire, Derbyshire and Lincolnshire, while membership is growing fastest in places such as Wiltshire and East Sussex, outside the party's traditional heartlands.
Wikileaks said this morning: "The lists have been verified to be accurate in all cases examined by Wikileaks, however it should not be assumed that every person with a BNP membership number is a current member of the BNP."
The party's leader, Nick Griffin, said he could neither confirm nor deny that the list was up to date.
"The allegation is that it is complete as of April this year," he said. "We have no way of knowing that as yet, but it might be possible that the new list is a concoction of the old leaked list combined with inquiries up until April. As everyone knows, inquiries are often made out of curiosity or even hostility, and cannot be taken as an indication of membership or support."
He said the leak was part of a "hysterical media-driven anti-British National party campaign which is growing in intensity as the party threatens the corruption and treason of the old Westminster parties".
The BNP's leadership was alerted to the leak yesterday and said it appeared to have been timed to undermine the party ahead of the appearance of its leader, Nick Griffin, on Question Time on Thursday.
The far-right map of Britain: BNP membership by constituency The publication of the list represents the third significant time the details of the BNP's membership have been made public. In November 2008, a list of members' names, contact details and in some cases jobs and hobbies was leaked by disgruntled members said to have become frustrated that the party had become too soft under Griffin.
In December 2006, an undercover investigation by the Guardian revealed that the organisation's members included Simone Clarke, then a ballerina for the English National Ballet.