Sky Broadband has begun blocking access to file-sharing site The Pirate Bay.
It follows Virgin Media and Everything Everywhere which have already taken similar action.
The High Court had demanded the move after complaints by the British Phonographic Industry (BPI) that TPB facilitated copyright infringement by providing magnetic links to movies, music and other media.
O2 and Talktalk said they were still working to implement the ban.
A sixth operator, BT, has been given extra time to make the necessary arrangements. It is expected to act within the next fortnight.
A statement from Sky said: "We have invested billions of pounds in high-quality entertainment for our customers because we know how much our customers value it. It's therefore important that companies like ours do what they can, alongside the government and the rest of the media and technology industries, to help protect their copyright."
A spokesman noted that it had acted ahead of a 1 June deadline.
This is the second court order of its kind that Sky has complied with following its block on Newzbin 2 in December.
The High Court issued different time limits to the different ISPs.
O2 has until 13 June to act, by which time it said it would block access to TPB's main site as well as other IP addresses that the BPI successfully claimed had been set up to enable access to the service.
However, the Torrentfreak news site has reported that TPB has since set up a new IP address giving access to its contents. It added the site was willing to play "an extended game of whack-a-mole" in which it would publicise new locations every time the courts ordered one of its addresses to be blocked.
A spokesman for the BPI said it was working with ISPs and the courts to ensure that existing orders were effective, but would not comment on whether it would seek to block further addresses.
Meanwhile, O2 is set to return to the High Court on Thursday for a hearing into a separate copyright complaint.
A judge will hear evidence in a dispute with Golden Eye International, a limited company which trades as Ben Dover Productions making pornographic films.
In March the firm won an order demanding O2 release details of thousands of its customers whose IP addresses it said had been linked to illegal downloads of Ben Dover's films.
At the time O2 said it had no option but to "co-operate fully".
The hearing is for the court to "approve the form of a letter" that Golden Eye wishes to send to its customers.
"In our first letter we seek to find out more information regarding evidence of an infringement of our copyright," Julian Becker, director of Golden Eye told the BBC.
"Depending on the response to our letters we will then decide our next action.
"Fundamentally we are pursuing those that are uploading not downloading. In effect these violations are unauthorised distribution, we are not pursuing those who have simply downloaded one film."
Mr Becker added that he was awaiting guidance from the court as to how much compensation his firm could seek.
Golden Eye previously said it wanted £700 for each infringement - a sum watchdog Consumer Focus described as "unsupportable".