An executive order signed by the president would cancel collective bargaining for hundreds of thousands of workers, the largest federal employees union said. The union was preparing legal action.

The White House cited national security concerns for terminating workers’ ability to bargain collectively, but the order applies at agencies with both direct and indirect links to national security.

President Donald Trump has signed an executive order aimed at limiting unionizing and terminating collective bargaining rights for government employees involved in national security-related work, the Office of Personnel Management (OPM) said Thursday.