Virgin Media has said it was forced to take its website offline for an hour during a hack attack.
Twitter feeds associated with the Anonymous collective announced: "Virgin Media - Tango Down #OpTPB".
The messages suggest that the attack was organised to protest against efforts to block access to The Pirate Bay's (TPB) file-sharing pages.
Virgin Media began preventing access to TPB last Wednesday following a High Court order.
Four other internet service providers - Sky, Everything Everywhere, TalkTalk and O2 - have also been ordered to prevent their users being able to visit TPB by this coming Friday. A sixth ISP, BT, has requested "a few more weeks" to consider its position.
Tweets issued by accounts linked to Anonymous also claimed TalkTalk was targeted over the weekend, although the network could not confirm the details.
A statement by Virgin Media said that the distributed denial of service (DDoS) attack lasted one hour, beginning at 5pm BST.
It added that it was only blocking TPB because it had been forced to do so.
"As a responsible ISP, Virgin Media complies with court orders but we strongly believe that tackling the issue of copyright infringement needs compelling legal alternatives, giving consumers access to great content at the right price, to help change consumer behaviour," it said.
Copyright defenders, including the British recorded music industry body BPI, have argued that illegal copies of films, books and music made available on file-sharing sites destroy creative industry jobs and discourage investment in new talent.