The BNP is bracing itself for potentially fresh embarrassment tomorrow when details of the party's rank and file UK membership are expected to be posted on the internet.

The list, which purports to be a snapshot of the party's support in April this year, includes the names, addresses, postcodes and telephone numbers of people who have signed up to the far-right group, including the grade of membership assigned by the party – standard, family, family plus, gold, OAP, and unwaged.

This list was leaked to a website, which insisted today that it was genuine, and that it intended to publish the information tomorrow. Today the BNP seemed unaware of the potential disclosure 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.

Today the political row about the BBC's decision to invite him on to the programme intensified, with the corporation refusing to bow to pressure from a cabinet minister to cancel Griffin's appearance.

The apparent disclosure of a membership list will add to the controversy surrounding the party. The Guardian has seen the list, but could not verify its authenticity. It appears to show that:

• The BNP had 11,560 members as of April this year.

• The party appears to have benefited from a surge in female recruits – one in eight party members are now women.

• The highest concentrations of members are in Leicestershire, Lancashire, Derbyshire and Lincolnshire.

If the document is authentic, it will be the third time the party's membership data has been made public in recent years. In November 2008, a list of members' names contact details and sometimes jobs and hobbies was leaked by disgruntled members said to have become frustrated that the party had become too soft under Griffin. That list was widely circulated on the internet and last month 37-year-old former BNP member Matthew Single, appearing before Nottingham magistrates, admitted leaking the data and was fined £200 for breaching the Data Protection Act. 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.

A spokesman for the website which is proposing to publish the list said today it wanted to reveal it in order to provoke a debate in the UK about why people want to become members of the BNP.

A BNP spokesman said it considered any publication of its internal membership data to be a criminal offence under the Data Protection Act. "This is an example of the perpetual hatred and vilification we face," said BNP deputy chairman Simon Darby.

Yesterday the Welsh secretary, Peter Hain, called on the BBC to withdraw Griffin's Question Time invitation because the BNP is currently "an unlawful body" after the party told a court last week it would have to amend its whites-only membership rules to meet discrimination legislation.

The last time BNP data was published, it emerged that teachers, policemen and former members of the Conservative, Labour and the Lib Dem parties had signed up. Several dozen admitted to being members and were named in the press. The latest list suggests that at least 19 of those members have now left the party.

Meanwhile, former military chiefs today warned that the armed forces were in danger of being hijacked by far-right groups. The Times reported that former generals had written a letter warning that political extremists had no right to share the armed forces' proud reputation.

The letter, signed by former heads of the army, General Sir Mike Jackson and General Sir Richard Dannatt, among others, said far-right groups were "fundamentally at odds" with the values of the British military.

The move follows the BNP's tactic of using images of Winston Churchill and wartime insignia during recent European election campaigns.

The letter reads: "We call on all those who seek to hijack the good name of Britain's military for their own advantage to cease and desist."

• This article was amended on 20 October 2009. We said that the purported BNP membership list includes one peer. This was wrong. It arose from confusion over similar surnames. This has been corrected.