The Transportation Security Administration has ordered airports to send all vendor employees through security checkpoints before letting them go to work in restaurants or shops in the secure areas of terminals.

The agency had resisted congressional calls for such a requirement, with TSA acting administrator David M. Stone arguing as recently as March that extensive background checks were sufficient to catch untrustworthy employees.

Many airports already screen some workers but complain that requiring the practice could cause further delays at crowded checkpoints and interfere with each airport's managing of its own resources.

But the TSA issued three security directives Tuesday evening that require airports to come up with a plan to screen all employees going into secure areas and to limit the number of doorways that provide access to "sterile" zones. The plans must be submitted to the TSA for approval within 30 days, airport officials said.

A TSA spokesman declined to comment on the specific directives, which are considered sensitive and not for public release. But agency spokesman Mark Hatfield Jr. said in an e-mailed statement that the agency "continually analyzes threat information and evaluates the system of security layers we have placed at our nation's airports. . . . As part of these newly issued security directives, the TSA will require enhanced background checks and improved access control for airport employees working in restricted areas."

Rep. Peter A. DeFazio (D-Ore.), who has pushed the TSA to take such action, said yesterday he was pleased with the directives.

"It's a good step," said DeFazio, who raised the issue 16 months ago after visiting a Detroit airport and witnessing vendor employees going in and out of the sterile zone without passing through security checkpoints. Even though all such workers are required to pass criminal background checks, DeFazio worries that they could be bribed to carry weapons into terminals and slip them to passengers who have already gone through airport metal detectors.

"There's not a downside to adding this level of security, except some possible delays or inconvenience until they get the screener workforce level sorted out," DeFazio said.

TSA spokesman Nico Melendez said passenger delays should be minimal because airports have already designed their work schedules so that worker shift changes occur at off-peak travel times to avoid clogging checkpoints.

Baltimore-Washington International Airport already requires employees to go through checkpoints before entering the secure part of the terminal, a spokesman said. Some workers at Reagan National and Dulles International airports pass through checkpoints on their way into the terminal, but not all, said a spokeswoman for the Metropolitan Washington Airports Authority.

Some airports are concerned about employees who must go back and forth from secure to non-secure areas several times during the day, and whether they have to stop each time for screening, said Carter Morris, vice president for transportation security policy for the American Association of Airport Executives.

He said that his group has been discussing the issue for several months with the TSA and congressional staffers and that flexibility is key to making the directives work -- especially during the peak summer travel season. "It's something that is going to very definitely need to be worked out on an airport-by-airport basis," Morris said.

Rep. Peter A. DeFazio had pushed for security changes out of fear that airport venders could be bribed.