U.S. airlines on Saturday will begin asking travelers to provide their birth date and sex for the first time under a new aviation security requirement, federal officials said Wednesday.

The change comes as the Department of Homeland Security takes over responsibility for checking airline passenger names against government watch lists. The additional personal information, which airlines will forward to the Transportation Security Administration, is expected to cut down on cases of mistaken identity, in which people with names similar to those on terrorist watch lists are erroneously barred or delayed from flights.

U.S. airlines on May 15 started asking passengers for their full name as it appears on a government-issued identification card, a change intended to allow companies to upgrade their reservation and information systems. Starting Saturday, airlines will be required to get both the name and the additional information, although TSA is working with individual airlines to phase in compliance, TSA spokesman Greg Soule said.

Passengers should not be concerned if their airline does not ask them for the information, Soule said. The agency hopes to vet 100 percent of domestic passengers by March 31 and all passengers on international flights to, from or over the United States by the end of 2010 -- a total of 2 million daily passengers.

For now, there will be no penalty for passengers who do not provide the information, Soule said. However, once the program is fully implemented, they could be denied boarding passes, he said.

"We have been assured that no passenger will be turned away or be denied the ability to travel," said David A. Castelveter, a spokesman for the Air Transport Association of America, a domestic airline trade group. "It would simply mean if you didn't have the information, you would be subjected to secondary screening."

The TSA seems to have softened its stance since October, when then-TSA Administrator Kip Hawley and Homeland Security Secretary Michael Chertoff announced the Secure Flight program. They said that except in rare situations, passengers who did not provide the additional information would be denied boarding and subject at minimum to being flagged for additional screening at airport security checkpoints.

Citing security reasons, the TSA would not say how many or which airlines are ready to comply with Secure Flight. However, an aviation industry official said that starting Saturday a majority of domestic travelers can expect to be asked for the added information.

Full implementation of Secure Flight would fulfill a top aviation security goal after the Sept. 11, 2001, attacks. It was included in a 2004 law overhauling U.S. intelligence agencies.

U.S. officials said that by taking over watch-list vetting, the government will consistently apply the latest list information and sophisticated algorithms to catch name variations, and avoid the security risk of giving such data to industry.

Adding full names, gender and birth dates will allow 99 percent of travelers to avoid delays -- or all but 2,000 passengers a day, they said.

Civil liberties groups have said the government still lacks adequate redress procedures for people mistakenly matched to watch lists.

Watch-list mismatches have delayed countless passengers whose names are similar to those on the agency's no-fly list, or on a second list of "selectees" identified for added questioning. Travelers who are stopped often endure lengthy questioning without an explanation.

Watch-list mismatches have ensnared infants and toddlers and Sen. Edward M. Kennedy (D-Mass.). The wife of former senator Ted Stevens (R-Alaska), Catherine, was stopped after a computer flagged her name because of its similarity to "Cat Stevens," the pop singer who converted to Islam and took the name Yusuf Islam. The government said it placed him on the no-fly list out of concern over his donations to groups that it said might have terrorist ties.