Film-makers are going to court in a bid to block access to a site that hosts pirated versions of popular movies.
In a UK legal first, the Motion Picture Association (MPA) has applied for an injunction that would force BT to cut off customers' access to Newzbin.
The MPA, the industry body for a number of movie studios, said it was targeting BT first as the largest internet service provider in the UK.
BT confirmed it would be in court later but did not make any further comment.
The MPA wants BT to block Newzbin with the same system that stops access to sites hosting child sex abuse images.
The members-only website aggregates a large amount of the illegally copied material found on Usenet discussion forums.
The MPA is the international arm of the Motion Picture Association of America - the industry body representing movie studios such as Warner, Sony, Fox, Disney and Paramount.
It brought its action against BT because, as well as being the largest ISP in the UK with more than 5.6 million customers, BT supplies the site-blocking system known as Cleanfeed to many other big UK ISPs.
Success in the courts may mean the blocking spreads to those other operators.
Massive scale
The MPA began its legal action against UK-operated Newzbin in 2010, which resulted in the High Court telling the site to remove material it hosted that infringed copyright.
Newzbin went into administration soon after but its assets, including web domains, were sold to new owners and a fresh version of the site has popped up operating out of the Seychelles.
"Newzbin has no regard for UK law and it is unacceptable that it continues to infringe copyright on a massive and commercial scale when it has been ordered to stop by the High Court," said Chris Marcich, European president of the MPA, in a statement.
"We have explored every route to get Newzbin to take down the infringing material and are left with no option but to challenge this in the courts."
Court-imposed blocks have been used widely throughout Europe but a success for the MPA would mark the first time the tactic has worked in the UK.
The UK's Digital Economy Act does require ISPs to help rights holders identify users who may have downloaded music, software and videos illegally. However, it stops short of giving rights holders legal powers to pursue alleged pirates.
In a statement BT would confirm only that it would be appearing in court on Tuesday "following an application for an injunction by members of the MPA".
The Internet Service Providers Association said it would not comment until the court had made its decision.
