Government-led attempts to impose porn-blockers on new broadband connections has also blocked sex education websites, advice for people suffering domestic abuse and forums for people struggling with addiction… to porn.
The filters are designed to protect young people and families from ""stumbling across hardcore legal pornography", Prime Minister David Cameron said last week.
But report by the BBC's Newsnight showed that the four big ISPs which have rolled out their filters have all ended up blocking access to legitimate, mainstream sites.
BT's filter - which launched this week and forces users to opt in or out when they turn on their service - is one of the main offenders identified in the report.
Among the sites currently blocked by BT include Reducing The Risk, a site designed to help victims of domestic abuse. A local domestic abuse hotline has also been blocked in Doncaster, while Sexual Health Scotland has also effectively been taken offline by the filter.
In a wider test, TalkTalk was found to have blocked sites including Bish UK, a sex-education site, and a women's rape centre in Edinburgh, while leaving 7% of porn websites tested by Newsnight open.
Sky's filter was found to have blocked about 99% of porn sites, but also blocked six sites set up specifically to help people addicted to pornography.
A £25 million advertising campaign will begin in 2014, funded by ISPs, to try and explain the filters. Meanwhile the prime minister has said he plans to extend online blocks to "extremist" sites - further worrying campaigners for online freedom that the government has too much control over the web.
All of the companies identified by the Beeb said that their filters would improve over time:
TalkTalk: "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."
Sky: "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."
BT: "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."