Internet provider TalkTalk is to force users to choose whether they want to block pornographic sites, leaving BT and Virgin as the only major internet service providers not to have filters for adult material turned on by default.

TalkTalk will apply a block to all users' accounts, unless they have already opted out of such blocks. Customers will then have to ask TalkTalk to allow pornographic websites to be visited through their internet connection.

But users will be presented with a tickbox to allow them to turn the filter off, the first time they head to a site that has been blocked. The company call that approach an "unavoidable choice" — meaning that rather than applying the filter in a blanket style, it will instead force all users to choose whether they want it installed or not.

It says that 95% of users have already made a choice, many after being faced with such unavoidable choices.

A TalkTalk spokesperson said: "We have been working hard on building this new pop up to reach these last customers that haven't yet engaged with our market-leading filtering service HomeSafe.

"With the vast majority of our customers having now made their unavoidable choice to turn on parental controls, we are also committing that we will ask customers to re-confirm their settings once a year."

The filter is enabled by TalkTalk’s HomeSafe product, which is provided free to customers and can stop internet connections from being used to access violent content and gambling websites, as well as pornography.

Sky said on Wednesday that its Sky Broadband Shield, the filtering software that is provided to all of its customers, would be automatically switched on unless users had explicitly opted out.

The two other major internet service providers, BT and Virgin, have not yet introduced the filter.

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BT is contacting its customers to ask them whether they want to have the tool introduced. Virgin gives parents the option to turn on the free tools but keeps them off by default.

The tool sold as a way for parents to keep children from viewing graphic websites. It has been promoted by politicians including David Cameron, who has said that internet companies must do more to protect children from accessing internet pornography and other adult material.

But when such tools were optional, take-up remained low. A report from Ofcom in July 2014 said that on average only 13 per cent of new internet users opted to turn on the filtering software.