Plans for a national ID card scheme risk changing the relationship between the British state and its citizens, the information watchdog has warned.
ID card plans have proved controversial
Richard Thomas said he had initially greeted the plans with "healthy scepticism" but the details had changed his view to "increasing alarm".
The government hopes a pilot scheme will pave the way for compulsory identity cards within the next decade.
Mr Thomas told MPs the scheme was
"unprecedented" in international terms.
The information commissioner's warning came as part of the Commons home affairs select committee's inquiry into the draft legislation on identity cards.
Mr Thomas was essentially delivering a preview of his formal critique of the law plans, which he hopes to give to ministers in mid-July.
His comments will come as a blow to Home Secretary David Blunkett, who has faced opposition over the idea of compulsory cards inside the cabinet from ministers such as Jack Straw and Patricia Hewitt.
BBC political editor Andrew Marr said Mr Thomas had used strong language which ministers could not just brush aside.
But the home secretary's spokesman accused Mr Thomas of "a bit of grandstanding".
The commissioner keeps checks on the UK's data protection and freedom of information laws.
He said he was not opposed to identity cards in principle.
But he was worried the British plans were more comprehensive and ambitious than any other scheme in the world.
Mr Thomas told the MPs: "This is beginning to represent a really significant sea change in the relationship between state and every individual in this country."
It was now clear the scheme was not just about identity cards but about a national identity register, he said.
"It is not just about citizens having a piece of plastic to identify themselves.
"It's about the amount, the nature of the information held about every citizen and how that's going to be used in a wide range of activities."
The government had addressed worries of "function creep" - the plans going further than initially envisaged - in terms of the information held about people.
But there were still concerns the plans were too "open ended" about how the information could be shared between different agencies.
Last month, 10,000 volunteers started trialling the cards, which will include biometric details such as iris scans, fingerprints or facial dimensions.
From 2007 all new passports and driving licences will include biometric data and there will be separate identity cards for those who do not drive or have passports.
By 2012, it is estimated that 80% of workers will have the card or a combined driving licence or passport.
The Home Office hopes the scheme will be compulsory by 2013.
Mr Thomas questioned whether the so-called voluntary stage was truly voluntary because people applying for driving licences and passports would have no choice but to have the biometric cards.
The government hopes the scheme will provide a "gold standard" of identity and be a key tool in the fight against organised crime and illegal immigration.
But Mr Thomas said that if the ID cards did work out as the government planned they would be "a very, very attractive proposition for criminals".
He also warned that theft of cards or errors on the database could cause problems like people being denied access to public services or even loss of livelihoods.
Mr Blunkett's spokesman said the commission had not raised any of the issues with the home secretary directly, despite consultations.
"The information commissioner is responsible for data protection, but the
government has a much wider responsibility to balance civil liberties with
ensuring our security against terrorism, immigration fraud and organised crime.
"A modern ID card is an essential part preparing Britain for the 21st century
and we will be pressing ahead with our plans."