The ruling said the Office of Personnel Management (OPM) had told thousands of employees across a handful of federal agencies they were fired using authority the agency does not have, US District Judge William Alsup ruled.

"[OPM] does not have any authority whatsoever, under any statute in the history of the universe, to hire and fire employees at another agency," he said. "They can hire and fire their own employees." In court the government argued OPM asked, not ordered, the agencies to fire these employees.

They also argued that probationary employees are not guaranteed employment and that only the highest performing and mission-critical employees should be hired. Government workers have been the target of the Trump administration's efforts to reduce the federal workforce in the name of cutting costs. Many employees affected by the firings are probationary employees - usually those in their first or second year in a job - though they can sometimes be long-time employees who were recently promoted. There are some 200,000 probationary employees in the federal workforce.

Judge Alsup ordered OPM to revoke its directives to multiples agencies - including the Department of Veterans Affairs, the National Park Service, and the Defence Department - to terminate employees. He issued a temporary restraining order which puts a momentary pause on the government's actions until a judge revisits the matter in the coming weeks.