San Francisco, California - A federal judge on Thursday ordered the US government to reverse mass firings that are part of Donald Trump and Elon Musk's plan to slash the government's workforce.

The ruling directs the Office of Personnel Management (OPM) to withdraw directives sent to a number of federal agencies that resulted in thousands of staff being laid off.

"The Office of Personnel Management does not have any authority whatsoever under any statute in the history of the universe to hire and fire employees at another agency," US District Judge William Alsup said, according to The Washington Post.

"Congress has given the authority to hire and fire to the agencies themselves. The Department of Defense, for example, has statutory authority to hire and fire," he said at the federal court in San Francisco.

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The ruling is the latest legal blow to Trump's efforts to force through drastic changes in the US government.

It comes days after another district judge on the West Coast blocked his ban on refugee admissions, and weeks after a court suspended his executive order overturning the constitutional guarantee of birthright citizenship.

Thursday's ruling came after unions and advocacy groups sued over what they said were illegal orders that federal agencies fire all probationary staff.

A federal worker in the first or second year of their job is considered probationary, even if they have been promoted from a lower rank.

Tens of thousands of people were affected by the order.

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"OPM, the federal agency charged with implementing this nation's employment laws, in one fell swoop has perpetrated one of the most massive employment frauds in the history of this country," a plaintiffs' legal filing said, according to the Post.

"OPM lacks constitutional, statutory, or regulatory power to order other federal agencies to terminate employees who Congress authorized those agencies to hire and manage," attorneys said.