Broadband supplied by Sky will automatically stop customers from seeing pornography unless people actively opt out.
In a letter to its customers, it said that the Broadband Shield feature will be automatically enabled, unless people choose to change the settings.
Sky is the first broadband company in the UK to implement the policy.
UK prime minister David Cameron said two years ago he wanted mandatory online filtering of web sites to protect children.
Director of Sky Lyssa McGowan said the internet isn’t universally suitable for children.
In a blog on the Sky site, she wrote that it’s acting in the interests of security and online safety.
When dodgy sites are encountered by customers, a message will flash up on the screen and people will only be able to unblock specific pages by logging in and changing their settings.