Updated

Virtually all air travelers entering the United States beginning Jan. 23 will need to show passports — even U.S. citizens, the Homeland Security Department announced Wednesday.

Until now, U.S. citizens, travelers from Canada and Bermuda, and some travelers from Mexico who have special border-crossing cards for frequent visitors were allowed to show other proofs of identification, such as drivers' licenses or birth certificates.

"The ability to misuse travel documents to enter this country opens the door for a terrorist to carry out an attack," Homeland Security Secretary Michael Chertoff said in a statement.

Chertoff, who disclosed the effective date in an interview with The Associated Press on Tuesday, said the change was a crucial next step to helping ensure the nation's security.

Visit FOXNews.com's Homeland Security Center for complete coverage.

The department had been expected to institute the passport requirement for air travelers around the beginning of the year. Setting the date on Jan. 23 pushes the start past the holiday season.

"Each of these steps raises the bar to an attack. None of this is perfect. None of them is foolproof. But we're always better off when we build higher levels of security," he said in the interview.

"Right now, there are 8,000 different state and local entities in the U.S. issuing birth certificates and driver's licenses," Chertoff said. Having to distinguish phony from real in so many different documents "puts an enormous burden on our Customs and Border inspectors," he said.

In a few cases, other documents still may be used for air entry into the U.S. by some frequent travelers between the U.S. and Canada, members of the American military on official business and some U.S. merchant mariners.

Under a separate program, Homeland Security plans to require all travelers entering the U.S. by land or sea, including Americans, to show passports or an alternative security identification card when entering the U.S. starting as early as January 2008.

The Homeland Security Department estimates that about one in four Americans has a passport. Some people have balked at the $97 price tag.

The Sept. 11 Commission said in its report, "For terrorists, travel documents are as important as weapons."

The commission recommended strengthening security of travel documents. A 2004 law passed by Congress mandated the change to require passports as the only acceptable travel document, with few exceptions, but the exact date had been in question.

Canadian officials and some members of Congress from border states have expressed concern that the changes could interfere with travel and commerce.

Chertoff said his agency's data revealed that in September 2006, 90 percent of passengers leaving from Canadian airports had passports. The department estimated that 69 percent of U.S. air travelers to Canada, 58 percent of U.S. travelers to Mexico, and 75 percent of U.S. travelers to the Caribbean hold passports.

"Could James Bond and Q come up with a fake passport?" Chertoff asked, referring to the fictional British spy and his espionage agency's technical genius. Of course, he replied, because "nothing is completely perfect."

Still, he said, with new technology, it is increasingly difficult to forge passports, and having just one document to scrutinize should make inspection easier for both inspectors and travelers.

Get more coverage at the Homeland Security Center.