The updated ID has been required since May, but passengers without it have so far been allowed to clear security with additional screening and a warning. The Department of Homeland Security says 94% of passengers are already compliant and that the new fee is intended to encourage travelers to obtain the ID.

REAL ID is a federally compliant state-issued license or identification card that meets enhanced requirements mandated in the aftermath of the Sept. 11, 2001, terrorist attacks.

Obtaining the ID — indicated by a white star in a yellow circle in most states — means taking more documents to the motor vehicle agency than most states require for regular IDs. It was supposed to be rolled out in 2008 but the implementation had been repeatedly delayed.

Beginning Feb. 1, travelers 18 and older flying domestically without a REAL ID and who don’t have another accepted form of ID on them, such as a passport, will pay the non-refundable fee to verify their identity through TSA’s alternative “Confirm.ID” system.

TSA officials said that paying the fee does not guarantee verification, and travelers whose identities cannot be verified may be turned away. If approved, however, the verification covers a 10-day travel period.

The fee can be paid online before arriving at the airport. Travelers can also pay online at the airport before entering the security line, but officials said the process may take up to 30 minutes.

The TSA initially proposed an $18 charge for passengers without a REAL ID, but officials said Monday they raised it after realizing the alternative identification program would cost more than anticipated.

Other acceptable forms of ID include military IDs, permanent resident cards and photo IDs from federally recognized tribal nations. TSA also accepts digital IDs through platforms such as Apple Wallet, Google Wallet and Samsung Wallet at more than 250 airports in the U.S.