Last week the website of Chaos Computer Club, a European association of hackers who run the immense annual Chaos Communications Congress event, was blocked by parental filters on Vodafone, Virgin and Three. Although some access to CCC has been returned, if you are in the UK you still won’t have access to many sites, such as Pregnancy Sickness Support, Everlasting Flowers or HotWheels.com, depending on who your Internet Service Provider is.

Filters in the UK are default-on for mobile phones but are opt-in on ISPS- albeit they attempt to nudge parents into accepting. They may seem like a good idea, but in reality filters block much more than they are supposed to.

CCC suggested that they were victims to extremist blocking in the UK. However, it is likely that it was the use of the word ‘hacking’, a category several Broadband providers block for under-18s, that was the cause. CCC write about computer skills and education, much of it aimed at young people; it might be edgy, but it is highly debatable that it is adult-only.

Although the ISPs were told by David Cameron to set up filters to stop the smutty, what actually gets blocked can seem all too random, especially when some services don’t tell you why a site has been blocked. Had Vodafone provided a tool that told you why they block, we wouldn’t have to speculate on what happened to Chaos Computer Club.

Britain has many levels of web blocking and online censorship. Firstly, illegal child abuse images are blocked, based on lists produced by the Internet Watch Foundation. Then there are court orders which rule that specific websites (around 100) must be blocked, usually for copyright reasons. But the filters that affected CCC are those that block legal content. Introduced as ‘one-click to safety’ by David Cameron in 2013, the Government persuaded Internet Service Providers to implement filters, which block ‘adult content’ at the network level, and once opted-in there is one level of access per household.

Although that initial announcement was sold as preventing minors accessing pornography, they inevitably included a range of other categories: from alcohol and gambling to dating and blogging. Just a few weeks ago the Government put the pressure on to expand the filters again into ‘extremist material’. He said in his announcement, "In the UK we are pushing them [ISPs] to do more, including strengthening filters" proving once again that if a censoring mechanism is set up it will be used to enforce the Government’s world view.

Filtering is more complex than just switching off access to anything mildly questionable. This is where the randomness factor sets in. It’s left to the judgement of largely foreign companies, to whom the filtering task is outsourced, as to how to categorise sites. However, human communications cannot be easily divided into the ‘good’ and the ‘bad’. The result of ambiguity is ‘over-blocking’, where sites are filtered out through key word misunderstandings, and ‘under-blocking’, where actual porn is undetected.

The problem we face here is the lack of transparency over web filtering;  there is no oversight around blocking and no review process.

CCC is in a privileged position, despite being censored. With the support of advocates like BoingBoing and the media their site was switched back on by Vodafone on the 7th December, just days after reporting the problem. But smaller sites are less lucky, especially if they don’t know that they’re blocked. Each ISP applies their filtering differently. A business owner who has Virgin on her computer doesn’t know her website is being blocked on BT. If she’s not a customer of theirs she’ll have no easy redress to get the block removed. Even if she was a customer, they often unhelpfully suggest that she adds the site to her exclusion list – which is only effective on that computer.

What Open Rights Group is doing is seeking to expose the filtering, and campaign for free speech online. This year ORG volunteers built www.blocked.org.uk. Based on free software, the website allows you to type in a URL and check whether or not it is blocked by any of the UK’s mobile networks and most of the UK’s home broadband networks.

One particular concern was that because many charities work to respond to issues surrounding dugs, sex education, smoking, mental health and abuse, their websites would contain keywords likely to be blocked. Using Blocked we found that at least 54 registered charities in Scotland have websites that are blocked by one or more of the main UK ISPs.

These include the Say Women project in Glasgow which offers: "safe, supported accommodation and related services for young women, aged 16-25 years, who are survivors of childhood sexual abuse, rape or sexual.”A Dundee equalities project called Different Visions Celebrate that works with Under 25's "who have any issues or concerns due to their sexuality or the sexuality of a family member" was also blocked. We’ve even seen a lot of sites relating to feminism blocked, and in July 2014 Three blocked a piece on maternity leave in Jezebel.

As long as filtering is pushed out by the Government we will continue to see more and more websites censored. There needs to be a legal framework, not a behind closed doors policy in which David Cameron or Theresa May can add a new type of site to be blocked in the UK every week. If you would like to get involved, please share our Blocked tool, and find out if your website is blocked in the UK.