Washington DC - The US military will "immediately" start the process of removing some 1,000 transgender troops and will force out those who do not leave voluntarily by early June, the Pentagon said on Thursday.

President Donald Trump issued an executive order in January banning transgender military service, and the US Supreme Court ruled this week that the ban could take effect while litigation challenging it plays out.

"The Military Departments will immediately begin processing for separation service members who previously self-identified for voluntary separation prior to March 26, 2025," a memo from Pentagon chief Pete Hegseth said.

Approximately 1,000 troops who identified as having gender dysphoria diagnoses fall into that category, Pentagon spokesman Sean Parnell said in a statement.

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There were a total of 4,240 currently serving troops with such diagnoses as of late last year, according to a senior defense official, and the memo said those who do not voluntarily leave by June 6 for active-duty troops and the following day for reserves will be removed.

"On conclusion of the self-identification eligibility window, the Military Departments will initiate involuntary separation processes," the memo said.

In his January 27 executive order, Trump stated that "expressing a false 'gender identity' divergent from an individual's sex cannot satisfy the rigorous standards necessary for military service."

The Pentagon followed that up with a memo issued in late February stating that it would remove transgender troops from the military unless they obtain a waiver on a case-by-case basis, as well as prevent transgender people from joining.

Transgender Americans have faced a roller coaster of changing policies on military service in recent years, with Democratic administrations seeking to permit them to serve openly, while Trump has sought to keep them out of the ranks.