David Cameron, champion of civil liberties, is about to impose a National Identity Number on you

Having killed Labour's plans for a National Identity Card, David Cameron seems to be on the verge of introducing its little brother, a National Identity Number. This is the unavoidable implication of the reports in recent weeks that the government will cancel the UK Census after the one in 2011.

The Census provides vital data for important decisions and social studies but is expensive. And there is another way. The Nordic nations replaced their censuses with data from other government databases years ago. The result? The US, which has a similar system to us, reportedly spends $36 per head on its census while Finland gets the same data for 20 cents per head.

This is the reason why the Office of National Statistics has been working on a Beyond 2011 project. It seems a no brainer. But look a bit deeper and the difficulties and downsides start to mount.

The Nordic nations have been able to get the data they need from their databases as a result of decades of work developing and implementing systems that talk to each other. And they are only able to do this because every citizen is allocated at birth (or immigration) a National Identity Number - and is happy to use it in every transaction with the state.

The systems developed for all the different government agencies over decades make this approach possible. The NIN is the thing that links all the databases together. And the willingness of citizens to use it (in countries that trust the state a lot more than we do) makes the system work.

There are three important conclusions from this. First, it could take Britain decades to implement an effective alternative to the Census. If we abandon the 2021 Census in the meantime, we risk making some expensive mistakes. Second, if we plough ahead with the 2021 Census then the costs of the Census in the coming years are going to rise as we develop and implement new systems. Third, the whole thing depends on the government implementing, and citizens adopting, a National Identity Number.

This third point is where the politics come in. Having argued so stridently that a National Identity Card was a violation of civil liberties, will Cameron be able to bring himself to argue for a National Identity Number? Especially when the creation of the systems behind the National Identity Number will of themselves give the government much greater ability to pry into the lives of each one of us, and make it easy for any changed government to deploy National Identity Cards at the drop of a hat?

Ministers may not mention a National Identity Number, but if and when you hear them announcing the end of the Census remember - that is what it will inevitably mean.