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Broadband provider Sky will block adult content by default, unless users opt out, it has revealed.

The decision was announced and will be phased in over coming weeks.

In 2013 Prime Minister David Cameron put pressure on internet service provider [ISPs] to make online filtering mandatory, saying it was the best way to protect children.

His request caused controversy among politicians and the internet industry.

Since then most of the UK's ISPs have offered filtering software for parents concerned about what their children may be able to access online but few have offered this by default, opting instead to allow parents or other customers to turn the filters on if they want them.

Sky's Broadband Shield is designed to filter out content deemed to be unsuitable for children aged under 13. It has been offered as default to new customers for a year.

But now the firm has decided to also offer it to all its existing customers, some 5.3 million in total.

In her blog post, Lyssa McGowan, Sky's brand director, explained why it was changing its policy.

"What we're doing now is simply making sure that the automatic position of Sky Broadband Shield is the safest one for all - that's 'on', unless customers choose otherwise," she said.

In the next few weeks Sky customers who have not chosen to either activate or disable its Broadband Shield would be emailed "giving them the opportunity to make a decision one way or the other", she said.

Once activated, users will not be able to access a filtered site without altering their settings.

Jim Killock, executive director of the Open Rights Group (ORG), was dismayed by the news.

"Censorship should never be turned on by default," he said.

"ORG's Blocked project (www.blocked.org.uk) has shown that filters block all kinds of websites, including some that provide useful advice to children and young people. Customers need to understand the implications of filters before deciding whether or not they want them."

All the UK's big four ISPs - BT, Virgin Media, Sky and TalkTalk - offer filtering systems to help parents prevent their children viewing inappropriate material online.

They have said that they will make sure all customers are aware of the filters.

In October, BT started interrupting browsing sessions for customers who had not set up the parental controls asking them whether they wished to activate them but not obliging them to.

Most of the systems used by ISPs work at a network level, which means that all devices that connect to a home router will be subject to the same filtering system.

Andrew Ferguson, founder of broadband news site ThinkBroadband, said that parents should not rely solely on filters to protect their children from online nasties.

"As ever the filters don't block all unsavoury material so are not a replacement for parenting and the embarrassing questions all parents have to face," he said.