By Jan. 22, millions of Americans will either need a REAL ID or a passport to fly domestically.

The Department of Homeland Security confirmed Monday that millions of Americans will have to get new state identification cards to fly within the country as of Jan. 22.

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Without a new ID, American travelers will have to use a passport even when flying domestically.

The DHS said it will begin enforcing mandates outlined in the REAL ID Act that was passed by Congress in 2005. Passed in the wake of the September 11, 2001 attacks, the law was aimed at heightening the security of IDs used at airports.

“The Real ID Act requires that IDs and the process to obtain them are designed to ensure that the person using them is who the ID says they are, and that they are as difficult as possible to manipulate or counterfeit,” said Ned Levi of nonprofit air passenger advocate group Travelers United in a statement.

Through delays and extensions, the law was never fully enacted. But the administration of Donald Trump has made it clear that it wants the law to be enforced.

All states are currently transitioning to REAL ID-compliant identification cards. Only 28 states, though, are fully compliant. The DHS has granted extensions for 26 states, including California, the most populous state, and New York. Travelers from these 26 states have until Oct. 10, 2018 to receive a REAL ID.

“It is time that the remaining jurisdictions turn their commitments to secure identification into action,” the DHS said on its website.

Because of the timing, though, some are worried the enforcement might result in thousands of travelers being turned away from airports during the busy holiday travel season in November and December.

Even in the 28 compliant states, there are many people with a valid driver’s license that is not a REAL ID. These travelers will have to register and pay again for a REAL ID or use a passport to fly after Jan. 22.