The move follows a landmark High Court order on October 14 2011, which allowed the Motion Picture Association to impose blocks via BT on the members-only pirate website.
BT told the BBC that the block had come into force on 2 November and is based on software developed to prevent users from finding websites showing images of child abuse.
But a Newzbin2 work around could be allowing site members to slip in under the radar.
"Newzbin have offered their customers a client for over a month that they claim will bypass Cleanfeed," said a spokesman. "However we're not in a position to comment on whether that claim is true or not."
The Newzbin2 ruling was the first case in the UK to use section 97A of the Copyright, Designs and Patents Act 1988 to require ISPs to block access to infringing websites.
The Newzbin2 group told the BBC that 93.5 percent of its active UK users have downloaded the bypass client as well as attacking the BT action by claiming "censorship of the free web has begun."
"Newzbin2 shall go on," it continued. Its users shall continue to access the site and its facilities."
"Nothing has changed and [the MPA] have no change after paying millions of dollars in legal fees."
Last week BT was urged by the music industry to build on the October Newzbin2 precedent and block access to The Pirate Bay, another illegal download site that has already been blocked by a number of ISPs across Europe.
John Smith, General Secretary of the Musicians' Union and Deputy Chair of the Creative Coalition Campaign, said: "Now that the High Court has clarified the law, as a sector we need to keep up the pressure on these illegal sites. For too long The Pirate Bay has been allowed to attack the livelihoods of individual artists and session musicians. We hope that BT will voluntarily block this prolific, illegal site."