Baltimore, Maryland - A US judge ruled Thursday to restrict access to Americans' Social Security data for Elon Musk's so-called Department of Government Efficiency after unions raised concerns about the unelected billionaire's sweeping powers.

The order by District Judge Ellen Hollander bans DOGE staff from accessing data containing information that could personally identify Americans, such as their Social Security number, medical history, or bank records.

The Social Security Administration (SSA) can only give redacted or anonymized records to DOGE employees who have completed background checks and training on federal laws, regulations and privacy policies, Hollander ruled.

The order is the latest legal challenge to Musk and DOGE's drive to dismantle large swathes of the US state.

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It comes after SSA chief Michelle King reportedly quit in February over DOGE's requests to access Social Security recipient information.

The case was brought by a group of unions who argued that the agency had "opened its data systems to unauthorized personnel" from DOGE "with disregard for the privacy" of millions of Americans.

The American Federation of State, County, and Municipal Employees (AFSCME) praised the ruling as "important."

"Today's decision provides significant relief and is essential in halting DOGE's unlawful and dangerous overreach," the union of civil servants said in a statement Thursday.

DOGE staff must also delete any non-anonymized Social Security data in their possession since Trump took office in January, according to the order. They can only access Social Security data that personally identifies Americans in specific cases where approval is granted by a court.

In those cases, requesting access to personal information to "search for fraud or waste" would not be a sufficient reason.