Ed Vaizey, the communications minister, said yesterday that he wants UK internet service providers to filter sexually explicit content. By default, your internet connection would be restricted to only allow appropriate sites – unless you call your ISP and ask them to turn the filter off. There are many reasons why this is a bad idea.

First, how do you define "explicit content"? Private web filtering companies have been struggling with that problem for years. Should advice pages for teenagers that have frank discussions about sex and sexual health be filtered?

Then there's the issue of free speech – sooner or later, someone will try to use this filter to block politically sensitive sites. Claire Perry, MP for Devizes, gave the most telling quote: "We just want to make sure our children aren't stumbling across things we don't want them to see."

And let's not forget just how bad the public sector tends to be at managing IT projects, or how poor their digital security can be.

Those are all great reasons not to waste untold millions of pounds either creating a government "great firewall", or requiring ISPs to do the same. But here's the most important reason of all: it won't work.

Any "think-of-the-children" internet filter has a fundamental problem: if it's effective enough to actually block adult content, it will also be irritating enough that almost everyone will turn it off.

An effective filter would have to censor Flickr, which has a large amount of adult imagery. It has to censor every blogging platform: Tumblr, for example, has a whole swathe of porn blogs, and there are untold numbers of sex bloggers writing reams of explicit text. And it has to censor YouTube, particularly if 4chan decide to flood it with porn again. Facebook could probably be let through, thanks to its strong filtering policies – although right now, most mobile providers block it for under-18s anyway.

If an adult content filter allows those sites through, it fails. And if it blocks those sites, then hardly anyone will use it – and it fails.

Even if a vaguely effective filter, which only removes obviously pornographic sites, would be enough for the proposal's supporters, it's still doomed. Not only because keeping up with the untold numbers of adult web pages that appear every day is a Sisyphean task, but also because children are not fools.

Most children understand computers better than their parents. Bypassing a filter is quite literally child's play. It's easy to use proxies, Tor, BitTorrent, an open wireless connection somewhere down the street or any number of other techniques when you've grown up with a keyboard in front of you.

It only takes one kid to know how to evade a web filter, and suddenly every one of their friends knows too. And all of their friends. And all of theirs.

If you want to protect children, get their parents involved in what they're doing online – and for younger kids, the e-safety lessons that'll start in schools next year are a great idea. But a Great British Firewall would be expensive, corruptible, and ultimately useless.

• Tom Scott's website is tomscott.com; he's on Twitter at @tomscott.