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.

The source of the data remains unclear but it appears to show details of the BNP's members and supporters at 15 April this year, as well as data about members whose subscriptions to the party had lapsed.

A Guardian analysis of the data suggests the BNP had 11,811 members as of April, including 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 in a statement that the data was "a malicious forgery" made up of a list leaked in November 2008, the names of people who had inquired about the party more recently and "thousands of names of people with whom the BNP has had no contact whatsoever".

"We have no idea from where this information has been drawn," he said. "Some of it looks like random items drawn from a telephone book."

Griffin said part of the list looked like information taken from the home of a former BNP employee by police during a previous investigation. 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 Griffin on Question Time on Thursday.

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.