The data protection watchdog accused ministers of resorting to "Sir Humphrey" justifications for the ID card scheme which do not explain how ID cards would achieve the government's goals of tackling terrorism, illegal working and crime.
Speaking to MPs on the cross-party home affairs committee last week, Thomas said he was increasingly concerned about the privacy implications of the scheme and, in particular, the development of a national identity register to record personal information about 60 million people.
"The introduction of such a register marks a sea change in the relationship between the state and the individual," he said. "It is therefore essential that a clearly defined proposal is brought forward detailing how the scheme will work in practice."
Thomas warned that the population register was untested technology and could cause serious problems for individuals if data was lost or recorded inaccurately, ranging from loss of access to government services to loss of livelihood.
He urged the government to give individuals the right to access and correct mistakes on their data on the register - a right denied in the draft ID Cards Bill.
Thomas also warned that making the register a gold standard for verifying identity would make ID cards a magnet for criminals and counterfeiters.
The government had failed to address the risk of "function creep" in the way that it plans to use information stored on the register, he said.
The draft ID Cards Bill was too broadly worded and too open-ended, allowing officials to access personal information from the register for reasons which were not adequately defined, Thomas said. He also criticised ministers for describing the ID scheme as voluntary, when in practice people would not be able to opt out of receiving ID cards when they renewed their passports and driving licences.
The government has estimated the cost of the scheme at between £1bn and £3bn, but the cost of rolling out biometric card readers could push the figure up to £6bn.