Transgender individuals are to be banned from serving in the US military "in any capacity", US president Donald Trump has announced, citing "medical costs and disruption".
In a series of tweets on Wednesday, Mr Trump said he had made the decision after consultation with his generals and military advisors.
"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," he said on Twitter.
“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. Thank you.”
The decision will dismantle a key Obama-era policy. Former defence secretary Ash Carter ended the ban on transgender people serving in the military last year, allowing transgender people to openly serve.
According to some estimates, up to 4,000 transgender people may currently work in the US military.
Mr Trump's decision took Washington by surprise, but is likely to please conservative members of the Republican Party in Congress.
In recent months, a group of House Republicans have tried to block funding for gender transition surgeries in the military, introduced by Mr Obama, though this was voted down in the House earlier this month.
Representative Ted Lieu, a Democrat who served in the military, said the move was a "terrible decision", arguing that it would exclude qualified and competent people from the military and "hurt our military readiness".
In a tweet in June last year, Mr Trump appeared to signal his solidarity with the LGBT movement.
“Thank you to the LGBT community! I will fight for you while [opponent] Hillary [Clinton] brings in more people that will threaten your freedoms and beliefs,” he said in a tweet during the campaign trail.