Sky will automatically block porn and other "adult content" for new customers who join the internet provider from next year.
Currently the company's Broadband Shield filtering system is optional for customers. But from early next year Sky Broadband will set the block as a default for new subscribers.
It will also be programmed automatically not to show content unsuitable for those under 18 until 9pm, although users can personalise this setting to suit them.
Emails will be sent to all existing customers, asking them if they would like the filter to be switched on, Sky said.
Sky will automatically block porn and other 'adult content'
If the email is ignored, Sky's policy is to turn on the filter automatically, as it did following a similar email sent in January, when all customers who joined prior to November 2013 were contacted.
The policy is part of a system to create an "unavoidable decision", where those who do not want a filter must actively choose to say so - something suggested by David Cameron in 2013.
Lyssa McGowan, Sky's brand director for communications products, said: "We believe that this 'default on' approach will mean much greater use of home filters and ensure a safer internet experience for millions of homes.
"It came about as we looked for the best way to meet the Prime Minister's objective of providing more protection for children when they use the internet."
The Government supports Sky's plans.
Baroness Shields, the minister for internet safety and security, said: "Family filters have proven to be an extremely helpful tool for parents to safeguard children from age-inappropriate content.
"Sky's 'default on' approach is a great example of how industry is exploring different technologies to help keep children safe online."
According to figures from Ofcom, by June this year Sky customers were the most active when it came to using content filters, with more than 30% leaving content filters switched on, while none of its rivals - BT, Virgin Media and TalkTalk - had reached 15%.
Sky added that 62% of the customers it had sent the "unavoidable decision" email had kept some form of parental control turned on, the Press Association reports.
The move has been criticised in some quarters however, with the Open Rights Group, which campaigns for freedom online, suggesting that Sky was not "giving customers an informed choice about filters" by enabling them by default.
The group added that parents should not be lulled into a "false sense of security" by filters, and should talk to their children about how to safely use the internet.