Pornography filters used by major internet service providers are blocking websites offering sex education and advice on sexual health and porn addiction, the BBC has learned.
The four major internet companies have started to roll out so-called porn filters to their users.
BT launched its filter this week, Virgin has a pilot programme ahead of a full launch early in 2014, and Sky's was turned on a month ago.
TalkTalk's filter started in May 2011.
Last month, Prime Minister David Cameron welcomed "family-friendly" filters and said they were important to stop children "stumbling across hardcore legal pornography".
But BBC's Newsnight has discovered all the major ISPs that have launched full default filters are also failing to block hardcore porn-hosting sites.
All new customers will be prompted to decide whether to opt in or out, while existing customers of major ISPs will be presented with an "unavoidable choice" about whether to sign up.
Among the sites TalkTalk blocked as "pornographic" was BishUK.com, an award-winning British sex education site, which receives more than a million visits each year.
TalkTalk also lists Edinburgh Women's Rape and Sexual Abuse Centre website as "pornographic."
The company also blocked a programme run by sex education experts, and taught to 81,000 American children, that has been in development for more than 20 years.
TalkTalk's filter is endorsed by Mr Cameron but it failed to block 7% of the 68 pornographic websites tested by Newsnight.
Sky's filter fared much better, blocking 99% of sites, but it did block six porn-addiction sites.
In the new year the four major ISPs will fund a £25m advertising campaign to explain the filters and other aspects of children's safety online.
The filters were brought in following increased parental awareness of the ease with which children can access pornography online.
Victoria Shotbolt, chief executive of the Parent Zone, said: "It's great that the four ISPs have got together and are doing an awareness-raising campaign. But it isn't even starting to be enough.
"We're focusing so heavily on filters and all of the ISPs having them and public wi-fi having filters that the message getting through to parents is that those filters will do the job."
Justin Hancock runs BishUK and was not aware his site was being blocked by some filters until he was alerted by Newsnight.
He said: "It's really frustrating because I'm trying to provide a sex education site for young people and it's hard enough directing young people to good quality information on the internet.
"They might fix my site in the short-term but what about all the other sites that are out there for young people, not just sex education sites… who are TalkTalk to say what is allowed and isn't?"
The UK Council for Child Internet Safety has a working group to discuss over-blocking.
A TalkTalk spokesman said: " 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."
A Sky spokesman said: "We know that no one single technology currently provides all the answers. That's why we have a quick and easy way for misclassified sites to be unblocked. Any Sky home has the ability to fully customise their filters."
A BT spokesman said: "Categorisations are constantly updated to keep pace with changing content on the internet and we will investigate any concerns and make changes as necessary. BT Parental Controls can be customised to suit each individual family's needs."
Find out more about this issue on Newsnight on BBC2 at 22:30 GMT on Wednesday 18 December.