This Wednesday, I woke up to find an urgent email in my inbox from the Open Rights Group, letting me know that Lords Razzall and Clement-Jones, both Liberal Democrat peers, had introduced a web censorship amendment to Peter Mandelson's Digital Economy Bill.

I had a moment's confusion, because I assumed that the LibDems (a party I belong to) would have proposed a bill against web censorship. But no, our peers had put forward an amendment that would allow courts to order all of Britain's ISPs to shut off access to specific sites if these sites were found to be involved with copyright infringement.

Like many LibDems, I wrote to the Lords using WriteToThem and told them that promoting censorship – that is, shutting off entire swaths of the web because parts of a site infringed upon copyright – was not consistent with the values of the "party of liberty." So I was even more horrified to discover on Thursday that Razzall and Clement-Jones had withdrawn their amendment and entered a new one, jointly with the Tory Lords, that was specifically aimed at eliminating "cyber-lockers" (also called "web lockers") – services like Google Docs, YouSendIt, RapidShare and so on – that allow users to upload files that are too big to be attached to email, and send a private download URL to the recipient instead.

In a statement on Liberal Democrat Voice, Clement Jones defended his amendment, saying:

"Around 35% of all online copyright infringement takes place on non peer-to-peer sites and services. Particular threats concern "cyberlockers" which are hosted abroad.

"There are websites which consistently infringe copyright, many of them based outside the UK in countries such as Russia and beyond the jurisdiction of the UK courts. Many of these websites refuse to stop supplying access to illegal content.

"It is a result of this situation that the Liberal Democrats have tabled an amendment in the Lords which has the support of the Conservatives that enables the High Court to grant an injunction requiring Internet Service Providers to block access to sites."

Judging from the flood of outraged responses that followed, LibDem members aren't on board with this. And I'm among them. Web lockers are a critical piece of our internet life, and an attempt to ban them is worse than misguided; it's actively detrimental to the UK.

First, we must acknowledge that web lockers are useful for more than piracy. As our routine media files have increased in size - multi-megapixel images, home videos, audio recordings of meetings and so on - it's become increasingly difficult to use email to share data privately with family, friends and colleagues, because most email servers croak over really big files. For example, the sound editor for my podcasts uses a web locker to send me the mastered audiofiles for my review (and he's not the only audio person who relies on this; many's the time I've had an audiobook publisher send me an MP3 of an audiobook for review through a web locker).

There are plenty of personal uses too: my parents live in Canada and are always hungry for video of their granddaughter, but I don't want to make our home movies available on the public internet, so web lockers save the day for us. And when my immigration attorneys needed a mountain of scanned bank statements sent to their office for my application for permanent residence in the UK, a web locker made it easy to convey an encrypted archive to them.

There's no way to square this need for private file sharing with the entertainment industry's demand that all files be placed in the public sphere, where they can be inspected for infringement.

The reason web lockers are used for piracy is that they support privacy. A call to end web lockers is really a call to eliminate the public's ability to exchange personal information out of sight of the wide world. The only way you can be sure that someone isn't using a web locker to share a bootlegged movie is by shutting off my ability to privately send my mum a video of my toddler in the bath.

And separate from that, there's the infrastructural cost of establishing a Great Firewall of Britain in order to block access to web lockers. Developing a system whereby parts of the net can be shut off for all of Britain creates the possibility that someone will use the system to shut off the wrong part of the net. I'm not just talking about the danger of a hijacker breaking into the system to shut down or redirect traffic to legitimate sites (say, Microsoft Security Centre or the BBC), but the attractive nuisance presented by such a system. Once you create the facility to shut off parts of the internet that are implicated in civil disputes, how long will it be before people who've alleged a libel or are worried about a trade secret being not so secret are lobbying to have this turned to their aid?

Which isn't to say that this will actually stop infringement. File sharers have already demonstrated their ability to use the perfectly legal, widespread proxy services abroad to circumvent network blocks - ask any 14-year-old whose school network is censored by blocking software and I guarantee you'll get an education in how to evade this kind of thing. Which is great news if you're a pirate, but why should sound engineers, doting grandparents, and solicitors have to learn how to evade the Great Firewall in order to conduct their legitimate business?

It's not as if this hasn't been tried abroad. When South Korea and the US signed a Free Trade Agreement in 2007, Korea - a global powerhouse on the IT front - agreed to take major steps to lock down its Internet, including a prohibition on web lockers. Three years later, Korea has slipped badly in the global league tables and finds itself bringing more and more criminal sanctions against its young people.

"The party of liberty" needs to rein in these Lords, and not just because a failure to disavow them will cost the LibDems votes in May; but because this amendment is bad for the UK, bad for copyright, and bad for freedom.