Virgin Media has turned on a filtering system to help parents stop children seeing inappropriate material online.
The firm is the last of the UK's big four ISPs to turn on the government-mandated filtering system.
Initially only new subscribers will be asked whether they want the filters turned on or off.
Like other ISPs, Virgin has pledged to ask all its customers by the end of the year whether they want to use a filtering system or not.
All the UK's big four ISPs, who between them have more than 20 million subscribers, agreed to implement the filters following government pressure to limit children's access to adult material.
Virgin's system works at a network level which means all devices in a house which connect via its router will be subject to the same filtering system.
Called "Web Safe" the system currently blocks all access to sites featuring pornography, drug use, hate speech, violence, self-harm and suicide. Virgin is still working on a more flexible system that will let customers exercise more choice over which sites customers can and cannot reach.
Virgin has also produced a series of guides for parents, called Switched On Families, to help them set up and administer the filtering system.
Web Safe will work alongside software from security firm F-Secure that can be installed on laptops, tablets and smartphones to protect users when they are away from home.
The government's plan to get all big UK ISPs using filters has proved controversial as studies suggest the filters are not very effective. Some educational and charity sites that try to inform children about sensitive subjects have been blocked while other clearly adult-oriented sites are not filtered out.
The government is setting up a scheme to sites alert ISPs when they are wrongly blocked.