Britain's major broadband providers are "hijacking" web browsing sessions to force customers to make a choice on whether or not to turn on content filters. All of the country's leading ISPs now provide adult content filters, after the government threatened to legislate if they weren't offered to all customers.

As part of the deal struck with the government, the ISPs said all customers would be asked to make a decision on whether to switch the filters on or not. New customers are asked whether they want the filters switched on during the set-up procedure, but the ISPs are now having to take measures to force existing customers to make a choice.

BT is interrupting customers' surfing sessions by redirecting them to a page asking if they want the filters switched on. Only if customers make a 'yes' or 'no' decision are they allowed to continue surfing as normal, according to a report on Wired. Customers shouldn't see the message again once they've made their decision.

Sky and Virgin Media are doing likewise, although not preventing customers from continuing to browse other websites if they fail to make a choice. TalkTalk, which had its parental control filters up and running before any of its major rivals, says it won't be interrupting customers' surfing sessions, but will ask them to make a choice on the filters the next time they visit their My Account page.

The network-level filters offered by the ISPs have proven to be blunt tools for preventing children accessing adult content. The nature of the filters means that whole domains have to be placed off limits or permissable. Consequently, parents wanting to give their children access to Google can't prevent them from searching for pornographic photos on Google Images, for example.

It's also ridiculously easy to circumvent the filters, in some cases. Some providers fail to block access to popular proxy sites, allowing unlimited access to any website. Some also fail to block pornographic sites/images accessed via smartphone or tablet apps.

There have also been several instances of the filters being caught "over-blocking". TalkTalk's filter was found to be erroneously blocking off an award-winning sex education site and the website of a clinic for victims of rape and sexual abuse. BT's filters also blacklisted sites for domestic abuse helplines and sexual health education.