US President Donald Trump says 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.
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, 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 reversal will certainly anger transgender campaigners already alarmed at the delay on recruitment.