Porn filters introduced by ISPs following the Prime Minister’s calls for action to block ‘harmful’ online content are denying access to sex education websites and advice on sexual health, it is reported.

A BBC report last night claimed that measures being introduced by four major internet providers are failing to block hardcore porn while filtering out sites dedicated to helping those with porn addiction, or helping teenagers with sex education.

The Newsnight investigation revealed Virgin, BT, Sky and TalkTalk’s porn filters are all failing to do what was required of them.

David Cameron spent the summer pushing for ISPs to introduce porn filters automatically to households’ internet services, requiring the companies to provide only an opt-out option if they did not want such parts of the internet restricted.

Cameron said the measures were necessary to protect children from legal pornography.

TalkTalk was claimed to block BishUK.com, the award-winning sex education site, as well as Edinburgh Women’s Rape and Sexual Abuse Centre, said the BBC.

A spokesman told the corporation: " Sadly there is no silver bullet when it comes to internet safety and we have always been clear that no solution can ever be 100%. We continue to develop HomeSafe and welcome feedback to help us continually improve the service."