In a written statement to the U.S. Senate Foreign Relations subcommittee, the department said it had no idea Americans would be so willing to comply with the new law, The Washington Post reported Wednesday.

"In some ways, we drummed up business -- and more business than we anticipated," wrote Maura Harty, assistant secretary of state for consular affairs.

Willingness to comply with the new law has resulted in a backlog of nearly 3 million passport applications, forcing the State Department to relax this year's requirements for U.S. citizens traveling by air to Canada, Mexico and the Caribbean.

Travelers who have proof of application will be allowed to re-enter the United States without a passport until Sept. 30.

Harty told the Senate panel her department expects to receive more than 17 million passport applications this year and 23 million in 2008.