Broadband giant Sky will block "adult content" by default for all new customers, the company has announced.

Sky said the move would result in "much greater use of home filters", but customers could still opt-out if they wanted to.

In 2013, Prime Minister David Cameron said broadband users should face an " unavoidable decision " over whether to enable filters or not.

But the move has been criticised by internet rights campaigners.

Unavoidable decision

Sky launched its online content filtering product Broadband Shield in November 2013, but a year later only 3% of its existing customers had opted to switch it on.

In January 2015, Sky emailed customers who had joined prior to November 2013 and asked them whether they wanted to switch Broadband Shield on.

The filter was automatically switched on if customers ignored the email.

The company will now switch Broadband Shield on by default for new joiners, starting in early 2016. Existing customers that have not yet been prompted to opt-in will also be sent an "unavoidable decision" email.

'Keeping children safe'

The UK's biggest broadband providers have all agreed to present customers with an "unavoidable decision".

Sky has seen the biggest engagement with between 30 and 40% of all customers leaving the content filter switched on.

BT told the BBC it had contacted its home broadband customers, and displayed messages to web users until they made a choice about whether to switch on filters or not.

"In line with recommendations from the Mothers Union's Reg Bailey, we don't switch parental controls on or off ourselves. We believe this is more effective when it comes to keeping children safe," a spokesman said.

Telecoms regulator Ofcom said 6% of BT's customers had opted to switch on parental controls by June 2015, although take-up in households with children was 36% according to BT.

About 12% of Virgin Media subscribers and 14% of TalkTalk customers had switched on content filtering by June 2015.

Virgin Media told the BBC: "All of our customers have made an active and informed choice on whether to switch filters on or off."

Informed choice?

The Open Rights Group (ORG), which campaigns to protect internet freedom, had criticised Sky for enabling filters by default.

"Switching filters on by default, even if there are no children in a household, is not giving customers an informed choice about filters," said Pam Cowburn, communications director for the ORG.

"Parents should not be lulled into a false sense of security by thinking that switching on filters will automatically keep their children from seeing unsuitable content.

"Parents need to talk to children about their internet use and help to ensure that they have the skills they need to navigate the web safely."

Sky told the BBC a majority of existing customers had ignored its "unavoidable decision" email in January 2015, and had their content filter switched on automatically. But only 27 people had contacted its call centre about the plan.

"We think that default filtering is the best way for industry to meet the government's commitment to reduce children's exposure to inappropriate content," said the director of Sky Broadband, Lyssa McGowan.