WASHINGTON —  A federal judge Monday blocked the Defense Department from putting in place a merit pay system, saying it would erode bargaining rights for 650,000 employees.

U.S. District Judge Emmet G. Sullivan said the proposed workplace rules also would make it unfairly hard for employees to appeal unfavorable personnel decisions. The suit was filed in November by the American Federation of Government Employees and nine other federal employee unions.

Another federal judge last year barred a similar plan from taking effect at the Homeland Security Department.

Under the proposed system, Defense Department leaders, including Secretary Donald H. Rumsfeld and the secretaries of the various branches of the armed services, would be able to override provisions of collective bargaining agreements.

It would give Rumsfeld greater flexibility to change workers' assignments, allowing him to put civilians in some administrative jobs currently held by armed services members, who could then move to military positions.

Rumsfeld has said the changes are critical to improving the management of the department's civilian work force, which totals about 750,000. Some would not be affected by the changes.

Homeland Security officials have said they need the new rules to enhance their flexibility to respond to terrorist threats.