Los Angeles, California - A judge on Thursday ordered six federal agencies to rehire thousands of probationary workers who were fired as part of President Donald Trump's push to slash the size of the government.

Judge William Alsup said the justification of "poor performance" for mass lay-offs was "a sham in order to try to avoid statutory requirements," the New York Times reported.

Ruling on a lawsuit brought by employee unions, Alsup ordered the departments of the Treasury, Veterans Affairs, Agriculture, Defense, Energy, and Interior to reinstate anyone improperly fired.

"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 at a hearing at the US District Court in San Francisco.

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Since returning to the White House in January, Trump has taken an ax to the US government, cutting spending programs and firing tens of thousands of federal employees.

His actions have faced pushback from the courts, with several judges issuing injunctions intended to halt them.