The Pentagon seen from above, Arlington, Virginia, U.S., January 27, 2025. /VCG

A California federal judge on Thursday ordered six U.S. agencies to reinstate thousands of recently hired employees who lost their jobs as part of President Donald Trump's purge of the federal workforce.

The ruling by U.S. District Judge William Alsup during a hearing in San Francisco was the most significant blow yet to the effort by Trump and top advisor Elon Musk to drastically shrink the federal bureaucracy. Government agencies face a Thursday deadline to submit plans for a second wave of mass layoffs and to slash their budgets.

Alsup's ruling applies to probationary employees at the U.S. Department of Defense, Department of Veterans Affairs, Department of Agriculture, Department of Energy, Department of Interior and the Treasury Department.

The judge said the U.S. Office of Personnel Management, the human resources department for federal agencies, had improperly ordered those agencies to fire workers en masse even though it lacked the power to do so.

"It is a sad day when our government would fire some good employee and say it was based on performance when they know good and well that's a lie," said Alsup, an appointee of former President Bill Clinton, a Democrat.

White House press secretary Karoline Leavitt in a statement said Alsup lacked the power to issue the ruling and that the administration would "immediately fight back."

Alsup during the hearing said agencies can engage in mass layoffs but are required to comply with a number of legal requirements.

Probationary workers typically have less than one year of service in their current roles, though some are longtime federal employees. They have fewer job protections than other government workers but in general can only be fired for performance issues.

Alsup ordered the agencies to reinstate workers who were fired over the last few weeks, pending the outcome of a lawsuit by unions, nonprofit groups, and the state of Washington.

He did not order the 16 other agencies named in the lawsuit to reinstate workers, but said he would promptly issue a written decision that could expand on Thursday's ruling.