A High Court judge has ruled that BT must block access to a website which provides links to pirated movies.
Newzbin 2 is a members-only site which aggregates a large amount of the illegally copied material found on Usenet discussion forums.
The landmark case is the first time that an ISP has been ordered to block access to such a site.
It paves the way for other sites to be blocked as part of a major crackdown on piracy.
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."
If you're honest you just end up footing the bills for the thieves to make up for the lost revenues.
Rather than suing you, they ask ISPs to block you.
"It clearly shows that rights holders need to prove their claims and convince a judge to make a court order. BT has consistently said that rights holders need to take this route. We will return to court after the summer to explain what kind of order we believe is appropriate," the firm said in a statement.