President Donald Trump says transgender people cannot serve in the military.
He famously twitted that he had consulted with military specialists and foresaw “tremendous medical costs and disruption”.
The Obama administration last year allowed transgender people to serve openly in the military.
Later in June, the Defence Secretary James Mattis agreed to a six-month delay in the recruitment of transgender people.
Some Republicans have voiced opposition to permitting transgender people to serve at all.
The independent Rand Corporation estimated last year that about 2,450 of the 1.2 million active-duty service members are transgender.
Mr Trump said: “After consultation with my Generals and military specialists, please be advised that the u. s. Government won’t settle for or permit transgender people to serve in any capacity in the U.S. Military.
“Our military must be focused on decisive and overwhelming victory and can’t be burdened with the tremendous medical costs and disruption that transgender in the military would entail.”
The U.S. military’s ban on openly gay and lesbian servicemen and women – referred to as “Don’t ask do not tell” – was lifted in 2011.
Aaron Belkin, director of the Palm Centre, a leading think-tank which studies gender and sexuality in the military, told the BBC that Mr Trump’s decision would force transgender troops to in effect live as gays and lesbians did under “Don’t ask, do not tell”.
“Don’t ask, do not tell was a disastrous policy that crippled the military for nearly 20 years,” Mr Belkin said.
“It’s not clear why the president would need to bring it back now for transgender troops, when all the evidence suggests that inclusive policy benefits the military and discrimination hurts the military.”