Talk about an unwelcome arrival. The Government has unveiled a small piece of plastic which represents a big threat to our historic liberties. The Home Secretary, Jacqui Smith, was showing off Britain's first biometric ID cards yesterday. From November, the cards will be issued to foreign students from outside the European Union and to marriage visa holders. According to Ms Smith: "We want to be able to prevent those here illegally from benefiting from the privileges of Britain."

The hundreds of thousands of irregular migrants who work in Britain's black economy will find that reference to the "privileges of Britain" rather rich. The truth is that irregular workers tend to go to extreme lengths to avoid any form of contact with the British state. They do not consult doctors, complain to the police of routine mistreatment, or claim benefits because they fear being deported. Rather than looking for another way to make life more difficult for such people (the vast majority of whom are simply looking to better their lives in a country with a ready supply of work), the Government should be considering an amnesty, as proposed by Anthony Browne in this newspaper today.

That said, we should not be distracted from what is really going on here. ID cards are not about migrant workers, but about all of us. The Government is preparing for its planned nationwide roll-out of ID cards in 2011 by first imposing them on one of the most reviled groups in our society. It is the thin edge of the wedge.

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But we should not submit to such underhand tactics. The stupidity of forcing through this costly scheme as Britain enters an economic downturn should be obvious. The Government has stretched the public finances to the limit. To embark on a £4.7bn ID card project at such a time is fiscally irresponsible.

The public mood has also hardened since the Government first hit upon the idea. Who can have witnessed the carelessness with which the state has treated our personal data in the past year and still feel confident about handing over such sensitive information to officialdom?

Yet the most powerful argument against ID cards remains one of principle. The idea that we should routinely have to carry around a piece of card to prove who we are is an outrage. Even if Britain was still economically booming and the Government had an impeccable record of protecting our personal information, these cards would be unconscionable. Nothing has changed. We do not need ID cards and anyone who wishes to defend our freedoms should stand up and tell the Government so with a clear voice.