President of the American Federation of Government Employees (AFGE), Everett Kelley, called the new order an example of Mr Trump and Mr Musk's "utter disdain for federal employees and the critical services they provide to the American people".
Posting on X, Mr Musk, who is the world's richest man, wrote: "Consistent with President @realDonaldTrump's instructions, all federal employees will shortly receive an email requesting to understand what they got done last week.
Hundreds of thousands of federal workers, including some judges, court staff and prison officials, have now received the email with the following instruction: "Please reply to this email with approx.
"It is cruel and disrespectful to hundreds of thousands of veterans who are wearing their second uniform in the civil service to be forced to justify their job duties to this out-of-touch, privileged, unelected billionaire who has never performed one single hour of honest public service in his life," Mr Kelley said.
Elon Musk has issued an ultimatum to US federal workers - respond to an email justifying the work they completed this week or resign.
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