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.
As is often the case, the announcement came 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.”
But the measure will not go into effect immediately, as spokesperson Sarah Sanders told reporters at a White House press briefing.
The implementation policy has to be worked out, she said when asked if troops on battlefields would be immediately sent back to the United States.
This was “a military decision” said Mrs Sanders, adding that it is “not meant to be anything more than that”.