Controversial plans for identity cards have begun what is expected to be a rocky ride in the House of Lords.
The cards have already been criticised by a Lords committee
Opening the second reading debate, Home Office Minister Baroness Scotland said the cards could help prevent terrorism, crime and illegal immigration.
But Conservative spokeswoman Baroness Anelay of St Johns said the plans were intrusive and expensive.
By convention, peers do not vote against bills at second reading debates but there will be opposition later.
The Identity Cards Bill has cleared the Commons but with the government's majority slashed to its lowest since the election.
The first identity cards are due to be issued in 2008, under the government's plans.
They will carry biometric data, such as iris scans, facial images and fingerprints, with the information stored on a national identity database.
The House of Lords Constitution Committee report has criticised the lack of safeguards in the government's Identity Cards Bill.
It said the plans would fundamentally change the relationship between citizens and the state.
On Monday Lady Scotland stressed police would not have any new powers to demand proof of identity or make people produce ID cards in the street.
The bill specifically said people would not have to carry the cards with them, she said.
And there would also have to be a further vote of Parliament if it was to be made compulsory for everybody in the UK to own a card.
Lady Scotland said the cards offered "clear benefits", including ensuring people were entitled to access public services.
"I believe the time has come when we might perhaps accept that the introduction of the Identity Cards Bill is a wise and sensible, some would say commonsense measure to protect our identities," she said.
For the Tories, Lady Anelay said the scheme would be the most ambitious technology scheme ever seen in the UK.
If a virus entered the system, it could create and conceal multiple identities and cause "mayhem", she said.
"The fact is that biometric technology is fallible," said Lady Anelay.
One in six people could not have their biometric details taken for ID cards, she suggested.
Lady Anelay said ministers' plans were "not just impractical but unworkable" and argued the money would be better spent on providing more police, border controls and better public services.
Liberal Democrat spokesman Lord Phillips of Sudbury also opposed the bill.
He said: "What none of us wants, even a government I think, is a slippery slope, at the bottom of which broods an over mighty state where the privacy of the citizen is largely figmentary..."
But the plans received strong backing from former Metropolitan Police Commissioner Lord Stevens, who said law enforcement agencies wanted one single means of identifying people.
He warned: "The identification that's used by terrorists at the present time is as good as the ID that I have here in my pocket."
One man imprisoned last week for £1m in fraud had used no fewer than 130 different identities, said Lord Stevens.