Amid fears of violence toward members, the EDL said it will support vulnerable people. They also urged members to change their online shopping details after concerns were raised that they would be published on the internet.
Officials were forced to email supporters after the incident, which is understood to have occurred in recent days, apologising for the “attack”.
It remains unclear who was behind the hacking incident and whether it was a politically motivated attack or a party member with a grudge against the group. The EDL did not disclose to supporters which police force was investigating the breach.
In their email to supporters, officials confirmed that hackers had accessed their donor lists and the far right group’s payments systems, exposing customers who buy merchandise.
“As you may have become aware the English Defence League clothing site was recently attacked,” read the email, seen by The Daily Telegraph.
“This attack took two forms, firstly member’s names and addresses were stolen from the donation database (and) secondly the details of members purchasing items from the clothing site.
“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.”
The email confirmed that officials had contacted police.
“If, as an affected member you are concerned for your safety, contact the police,” they said.
“With regards to the criminal action against the EDL, the leadership is working with the police and it is being taken extremely seriously.
“There is currently an on going investigation and evidence is being collected.”
The breach comes days after the party was embroiled in controversy after asking American preacher Pastor Terry Jones to attend a demonstration in Britain.
Theresa May, the Home Secretary, had faced calls to bar Mr Jones from entering the country after he was reportedly invited to address the EDL at a rally in Luton in February.
The preacher sparked outrage when he announced plans by his Florida-based church to burn copies of the Koran to mark the anniversary of the 9/11 attacks on America.
The preacher said he would fight moves to ban him from coming to Britain, insisting he would be bringing a ''positive message''.
But the EDL later claimed it had not invited Mr Jones to speak at the event but he had approached them and they agreed in principle.
The weekend’s incident is similar to a security breach involving the far-right BNP in 2008, where the names, addresses and contact details of some 10,000 of its members were published online.
The list, which included details of some members' jobs, was reported to include serving and former police officers as well as members of the armed forces.
A spokesman for the EDL did not respond to inquiries for comment.