US President Donald Trump has declared that transgender people cannot serve in “any capacity” in the military.
He tweeted that he had consulted with military experts and cited “tremendous medical costs and disruption”.
The Obama administration decided last year to allow transgender people to serve openly in the military.
But in June, Defence Secretary James Mattis agreed to a six-month delay in the recruitment of transgender people.
The Pentagon said this was to allow the different branches of the military to “review their accession plans and provide input on the impact to the readiness and lethality of our forces”.
Some Republicans have voiced opposition to allowing transgender people to serve at all.
The independent Rand Corporation estimated in 2016 that 2,450 of the 1.2 million active-duty service members are transgender.
In a series of tweets, Mr Trump said: “After consultation with my Generals and military experts, please be advised that the United States Government will not accept or allow transgender individuals to serve in any capacity in the U.S. Military.
“Our military must be focused on decisive and overwhelming victory and cannot be burdened with the tremendous medical costs and disruption that transgender in the military would entail.”
The US military’s ban on openly gay and lesbian servicemen and women – known as “Don’t ask don’t tell” – was lifted in 2011.
Republican opponents of transgender people serving in the military include Vicky Hartzler, a congresswoman from Missouri, who wants transgender service members honourably discharged.
Others are opposed to the military bearing medical costs associated with transgender recruits, such as gender realignment.
