A High Court judge has ruled BT must block access to a website providing links to pirated movies.
This is the first time that an ISP has been ordered to block access to such a website.
The website, Newzbin 2 is a members-only website that aggregates a large quantity of illegally copied material from Usenet discussion forums.
In his ruling, Justice Arnold stated: "In my judgment it follows that BT has actual knowledge of other persons using its service to infringe copyright; it knows that the users and operators of Newzbin 2 infringe copyright on a large scale, and in particular infringe the copyrights of the studios in large numbers of their films and television programmes. It knows that the users of Newzbin 2 include BT subscribers, and it knows those users use its service to receive infringing copies of copyright works made available to them by Newzbin 2."
BT and the Motion Picture Association (MPA) are due back in court in October to work out how the blocking will work.
The MPA represents a number of movie studios, including Disney, Fox and Warner. They launched this legal action as a final attempt to close down Newzbin 2.
BT has said it will not appeal the ruling.
Will this pave the way for other sites to be blocked to help with the crackdown on piracy?