The agency, which warned states this summer not to trust the identification numbers as someone's form of personal identification, said the restrictions are intended to make sure the numbers are only used for tax administration.

People without Social Security numbers get Individual Taxpayer Identification Numbers, or ITINs, so the IRS can keep track of tax returns. Since 1996, 7 million of these nine-digit numbers, which begin with a nine, have been issued.

"About one-quarter of the ITINs issued for tax return purposes never actually find their way onto a tax return," said IRS Commissioner Mark Everson.

Seven states allow individuals to use an ITIN in place of a Social Security number when applying for a driver's license, according to a survey this fall by the National Conference of State Legislatures. California Gov. Arnold Schwarzenegger this month repealed one such a law, which would have allowed an estimated 2 million undocumented immigrant drivers to apply for licenses.

The IRS this summer sent letters to all 50 states reminding them that ITINs are intended for tax administration purposes and warning of potential security risks in accepting the numbers as a form of personal identification.

"We do not subject ITIN applicants to the same rigorous document verification standards as Social Security number of visa/passport applicants," the letter said.

"If your state is considering legislation to accept ITINs as proof of identity for driver's licenses, please alert your legislators to potential security risks. State-issued photo identification provides unrestricted access to most U.S. air and ground transportation systems and entry to public buildings," according to the letter.

As a result of the new restrictions, taxpayers needing an ITIN must submit their application with a completed tax return. Applications without proof of tax needs will be rejected, the IRS said.

The number of documents accepted as proof of identity to obtain an ITIN was winnowed from 40 to 13.

The IRS also changed the appearance of an ITIN certificate from a card to an authorization letter to avoid any similarities to a Social Security card.