Soldiers, police officers, teachers and doctors were in fear for their jobs last night after the entire membership list of the far-Right British National Party was posted on the internet.

More than 12,000 names, home addresses, telephone numbers and e-mail contact details were included in a major breach of data protection. The names and ages of schoolchildren with family memberships were disclosed. Some supporters were listed with comments such as "discretion required - employment concerns". A number had their hobbies recorded.

The BNP said that its members could be vulnerable to violent attack as a result. Nick Griffin, the party leader, issued a statement saying that the list was "essentially genuine". He said that it was an old membership list that had been "slightly modified".

Speculation was growing about whether the leak had come from a member angry about internal feuding or from an anti-racist infiltrator.

Simon Darby, the party's spokesman, said that a disgruntled former member was the most likely source. The BNP won a High Court injunction against a former member in April preventing the material being used.

Mr Darby said: "If we find out the name of the person who published this list, it will turn out to be one of the most foolish things they have done in their life. I wouldn't want to have done that - I wouldn't be sleeping very well tonight. We are worried because children's names are on the list. It is not information that should be in the public domain. We are always receiving death threats. No true nationalist would have put the lives and livelihoods of fellow nationalists at risk."

A leading lawyer said that members would not be able to sue either the party or the source of the leak. Hugh Tomlinson, QC, said that even though it was a clear breach of data protection law the legal remedies were sparse. If the information was taken by a disgruntled former member, then the party was not to blame. The person who leaked the list could in theory face criminal proceedings that could lead to a fine but not jail, he added.

Keiron Le-mar, a former soldier who served in Iraq with the Somerset & Cornwall Light Infantry, was appalled to discover that even his Service number was included in the entry. He told The Times that he joined the party while a soldier, although he was aware that the British Army "frowned upon" BNP membership. "I'm obviously disappointed," he said. "I work for an energy company. I'm worried about that because in my job I go round people's houses. I'm involved in all sorts of walks of life. I don't know how they would look on that. It could make the company look bad."

He added: "I joined the BNP because I feel they are the people that stand up for Britain as a country. I'm not for the racist views. To be honest, all I've ever got was letters asking for money."

The list describes the occupations of some members that are deemed to be sensitive or of use to the BNP, such as NHS doctor, teacher, journalist, vicar, company director, scientist, engineer or construction manager. Others are listed as public speakers. The list appears to include several former police officers. Peter Fahy, Chief Constable of Greater Manchester Police, said: "Membership or promotion of the BNP by any member of the Police Service is prohibited."

There is no ban on teachers, doctors and nurses joining the BNP, but its racist reputation is seen as incompatible with frontline public service.

Mr Griffin suggested that the timing was deliberate, coming before the European Parliament elections.

Mr Darby said that the BNP had contacted the owner of the website, which had been hosting the list since Sunday, demanding that it take the personal information down, although he conceded: "Once it's out, it's out."

The BNP has been accused of racism because of its whites-only membership policy. The party emerged after a feud inside the National Front and won electoral success by ditching its neo-Nazi image in favour of the nationalist anti-immigration rhetoric coined by the likes of Jean-Marie Le Pen in France. It knocked Labour into fifth place in the Henley by-election, won a seat on the London Assembly and has come second in local elections in Barking & Dagenham, Stoke-on-Trent and Dewsbury, West Yorkshire.

Gerry Gable, publisher of the anti-fascist magazine Searchlight, believes that the list is fresher than the one subject to the court injunction. A previous security breach happened two years ago when an undercover journalist exposed supporters including the principal ballerina Simone Clarke.