An English Defence League (EDL) database has been hacked and members' information stolen, according to reports.
The Mujahideen Hacking Unit claimed it had taken data from an EDL clothing website, which the group claimed to have hacked last weekend, the BBC reported.
The far-right EDL has now warned its members that names and addresses had been taken both from the clothing site and the organisation's donation database.
"The EDL would like to apologise for this security leak. The leadership is doing everything they can to understand how this occurred so it can never happen again," an EDL email seen by the Daily Telegraph read.
"With regards to the criminal action against the EDL, the leadership is working with the police and it is being taken extremely seriously."
The EDL said the police had been contacted and an investigation was underway.
The hack may have been inspired by the EDL's recent involvement in the furore surrounding American preacher Pastor Terry Jones' entrance into the UK.
Jones, who had caused outrage for his calls to burn copies of the Koran on this year's 9/11 anniversary, was due to attend an EDL rally in February. Subsequently the UK Government was called on to prevent his entry into the UK.
Data leaks have occurred amongst far-right organisations in the past. In 2008, the British National Party had a membership list published on the internet.
This was not the work of hackers, however, as it turned out to be a leak by an ex-senior member of the party, who was fined after he was outed as the offender.
The web is full of anti-West hacking groups who have gone after a number of political websites in the UK.
Earlier this year, an IT PRO investigation into website defacers found a number of local Government bodies had been hit by hackers with a strong anti-West message.
The EDL website was not up and running at the time of publication.
Ash Patel, country manager for UK and Ireland at Stonesoft, told IT PRO a hack on a database would require some decent hacking skills, indicating the effort on the EDL was not the work of amateurs.
"We've seen this recently with Stuxnet - it was another one that someone wasn't doing just for a laugh. It's taken a lot of skill," Patel said.
"But the skills that in yesteryear were perhaps way up at the top of the tree have now been much more automated and have become, for the average script-kids out there, much more accessible."
