The head of the Transportation Security Administration moved yesterday to block attempts to unionize as many as 56,000 airport screeners, saying granting them collective bargaining rights could jeopardize national security.

Officials at the American Federation of Government Employees, a public employee union trying to organize screeners at several airports, said the order signed by TSA chief James M. Loy was illegal and vowed to fight it in federal court.

The conflict represents the government's first labor-management clash at an agency that is to become a part of the Department of Homeland Security. Over the objections of the labor unions, Congress granted the Bush administration broad authority to set pay, hiring and work rules at the department, which is to begin operations Jan. 24.

"Fighting terrorism demands a flexible workforce that can rapidly respond to threats," said Loy, whose agency is in charge of airport security. "That can mean changes in work assignments and other conditions of employment that are not compatible with the duty to bargain with labor unions."

Loy signed the order in response to petitions filed by the AFGE with the Federal Labor Relations Authority seeking elections to designate the union as the exclusive representative of security screeners at Baltimore-Washington International Airport and New York's La Guardia Airport. He said the 2001 Aviation and Transportation Security Act that created the TSA gave him the authority to do so.

Mark Roth, general counsel for AFGE, said that law gave Loy great latitude in personnel decisions but not the power to stiff-arm unions on national security grounds. Roth also questioned the timing of the move, which comes weeks before TSA merges into the new department.

"Only the president would have this authority," Roth said. He added that the union would seek to overturn the decision in federal court. "His action is murky at best, and his timing is certainly objectionable."

TSA spokesman Robert Johnson said Loy acted because yesterday was the deadline to respond to the union's petition. He also said that since the mid-1970s, several administrations have excluded from collective bargaining workers in agencies such as Defense, Transportation and the Secret Service.

"It's not a new idea," he said.

Union officials said they would press on with efforts to unionize workers at BWI, La Guardia and other airports. Screeners' salaries range from $23,600 to $35,400 a year. Union officials said screeners have complained about not getting their paychecks on time, last-minute changes to their work schedules and being forced to work double shifts.

"These employees are on the job, worrying about things that they shouldn't be worrying about," said Diane Witiak, an AFGE spokeswoman. "We're not abandoning these employees. They're entitled to a union, and we're going to give them one."

Johnson acknowledged that the agency has faced challenges since it began a year ago, including late first paychecks to fewer than 1 percent of screeners and long hours for some employees. But the agency moved quickly to fix the problems, he said. "There have been some hiccups," Johnson said, "none of which have been allowed to go unresolved."

Some analysts said Loy's action undercuts the recent pledge by Tom Ridge, President Bush's nominee for homeland security secretary, to work with unions in getting the department up and running.

"The closet fear of a lot of Democrats was that this [new department] was a union-busting activity," said Don Kettl, a professor of public affairs and political science at the University of Wisconsin at Madison. "It certainly looks like a situation where the future of unionization for this workforce is in jeopardy."

Gordon Johndroe, a spokesman for Ridge, noted that 25 percent of the 177,000 employees moving into the new department are members of unions. Ridge still plans to work with them, he said.

. "[T]here is going to be continuing dialogue and discussion and working together as this new department is formed," he said.