By Barnaby Mason UK affairs analyst

Mr Clarke says he will listen to MPs' concerns about the bill

The identity cards bill is a battleground in a wider struggle between the defenders of civil liberties and what they see as an increasingly authoritarian government.

It is only one element in a huge programme of security and crime legislation scheduled for the next 18 months.

Two issues are most commonly cited in official justification. According to the Home Secretary (Interior Minister), Charles Clarke: "ID cards will help tackle illegal immigration as well as support the work of the law enforcement agencies in tackling the ever present threat of terrorism."

But the head of the civil rights group Liberty, Shami Chakrabarti, described the government's programme as: "More tough talk and bad law... revealing a chronic lack of respect for our democratic traditions."

The argument over identity cards may strike many foreign observers as peculiar, especially in continental Europe where they are long established.

French bemused

The French embassy in London seemed bemused at being asked whether anybody in France objected to identity cards. They were an accepted fact of life and they allowed you to travel in other European Union countries without a passport - that was the message.

In many European countries, people use an ID card for travelling

In Germany, everyone has to carry an ID card from the age of 16. A German diplomat told me: "Nobody thinks about it, nobody questions it... if you're in trouble, you just show it... we don't mind giving information if it's necessary."

In Britain, an opinion poll suggested that 80% of people supported the introduction of ID cards. But opposition is being voiced on multiple grounds.

The traditional objection of the free-born Englishman, as he would see himself, is summed up in the sentence: "Why should I have to prove who I am?"

That may seem a little incongruous in a country with an estimated 4m closed-circuit cameras monitoring the population.

But the independent Information Commissioner, Richard Thomas, recently highlighted the dangers of the state collecting too much information about its citizens.

Privacy issues

Mr Thomas said the phenomenon had "a strong continental European flavour", citing the example of communist east Europe and fascist Spain in the 20th century.

The government knows it faces another rebellion against the identity card bill among its own supporters.

The veteran Labour member of parliament, Gwyneth Dunwoody, said she was greatly disturbed, since "the history of police forces or governments holding every element or information about people's lives is that... they are used in some instances by governments for the worst possible reasons".

The bill limits the information that can be held on the ID card data base, the National Identity Register, to relatively few items - notably, name, date and place of birth, address, nationality, immigration status and physical characteristics.

The biometric cards will include fingerprints

New legislation would be needed to extend the list - for example to financial and medical information, employment history or criminal records.

The same would apply before any move to make identity cards compulsory. Attempting further reassurance, the government emphasises that in any event people would not have to carry one with them.

But every official justification meets objections from civil rights campaigners and others.

Blair's legacy

On fighting terrorism, they say that terrorists either have legitimate identification documents or can forge them; the problem is knowing who is planning an attack. The government has given no proper explanation of how ID cards will help.

On immigration, the critics say ID cards would be of no use in tracking down illegal immigrants or failed asylum seekers - except in a police state carrying out repeated identity checks on the whole population.

On fraudulent claims for welfare benefits, they argue that in nearly all cases people are lying about their health or economic circumstances, not their identity.

And so on.

Then there are practical objections about the cost and feasibility of another huge computerisation project, given a recent history of near fiascos in the public sector.

Richard Thomas complained of a lack of clarity, with the government changing its line about the exact purpose of identity cards. And one information technology consultant said that having no clearly set objective was a recipe for disaster.

"I wouldn't touch it with a bargepole," the consultant said, "because the business requirement hasn't been defined and they're bound to change it along the way."

What is clear, though, is that for Tony Blair the introduction of identity cards is a key part of establishing his political legacy before he steps down as prime minister. Cynics might say that is the real business requirement.