WASHINGTON — New border crossing rules to take effect at the end of the month will initially mean longer lines for those entering the country, including returning Americans, Homeland Security Secretary Michael Chertoff said Thursday. But he said the rules are necessary to prevent another Sept. 11-style attack.

Critics of the effort need to “grow up,” Chertoff said in an Associated Press interview.

Starting Jan. 31, a driver’s license and oral declaration of citizenship will no longer be enough to enter the United States for Americans and Canadians age 19 and older. People will have to present proof of citizenship, usually in the form of a passport or a birth certificate. A driver’s license is not proof of citizenship.

Chertoff said longer lines at the border in the early days of the new policy are inevitable. “Until people get the message, there will be some delays,” he said.

But he added that should change once people get used to the new system, and border agents will be flexible in applying the new rules at the beginning.

Not implementing these changes would be a tragic mistake, Chertoff said. “I can guarantee if we don’t make this change, eventually there will come a time when someone will come across the border exploiting the vulnerabilities in the system and some bad stuff will happen. And then there’ll be another 9/11 commission and we’ll have people come saying ‘Why didn’t we do this?”’

Chertoff bristled at criticism that such extra security may prove too inconvenient for those crossing the northern and southern borders.

“It’s time to grow up and recognize that if we’re serious about this threat, we’ve got to take reasonable, measured but nevertheless determined steps to getting better security,” Chertoff said.

Chertoff said he was surprised to learn that simply stating “I am an American” and showing an ID card has been sufficient to get back into the country and likened it to an honor system. “I don’t think in this day and age we can afford the honor system for entering the United States,” he said. “Regrettably, we live in a world in which people lie sometimes about their identity.”

More than 8,000 different documents have been used to enter the United States, including library cards and student IDs. The proof-of-citizenship requirement will greatly reduce the ability to sneak by border agents with fake papers, Chertoff said. Border agents will now only accept about two dozen types of ID. However, under the new system agents will likely be examining many more birth certificates, which pose their own counterfeit risks.

Also beginning in February, people can apply for a passport card that will be smaller than a regular passport but will include security features.

Even as border security gets tougher, the birth certificate requirement marks a setback for the agency’s effort to enforce a post-9/11 law called Western Hemisphere Travel Initiative, or WHTI, designed to “get control” of the borders by verifying the citizenship and identity of everyone entering the U.S. by land, sea, or air from Canada, Mexico, and the Caribbean.

In June, Chertoff delayed the law’s passport requirement for land and sea crossings until next summer. Congress has since pushed it back even further to June 2009, and Chertoff has been forced to settle for requiring birth certificates as proof of citizenship.

Even though the June 2009 deadline comes after the Bush administration has ended, Chertoff is adamant that the government needs to get moving on implementing the border security changes as well as other recommendations of the commission that investigated the Sept. 11, 2001, terror attacks.

The Bush administration envisions an eventual passport requirement for everyone crossing the border into the United States. Congress passed the travel requirements law in 2004 but is having second thoughts, particularly as northern-state lawmakers argue the passport requirement will hurt tourism and trade in their backyards.

“Secretary Chertoff’s comments that those objecting to the plan need to “grow up” indicates that the department still doesn’t understand the practical effects of DHS policies on the everyday lives of border community residents,” Sen. Norm Coleman, R-Minn., said in a statement. “We expect DHS to do everything in their power to ensure that any changes on January 31 pose as little inconvenience to the traveling public as possible.”

The law’s requirements for air travelers in 2007 was quickly followed by a massive backlog in passport applications, and some fear that will happen again this year as Homeland Security tries to go forward with the land and sea crossings.

Many Americans don’t realize that by presenting their state-issued driver’s license at the border, they have not actually proven they are U.S. citizens.

WHTI is the latest bureaucratic battleground in the Bush administration’s long effort to enact and enforce tougher national standards for identification, whether through passports, driver’s licenses, or immigration documents.