Donald Trump has banned transgender people from serving in the US military "in any capacity".
He announced the move in a series of tweets, saying he had come to the decision "after consultation with my generals and military experts".
The move reverses a policy began under Barack Obama and currently under review, in which the Pentagon ended a longtime ban on trans people serving openly in the military.
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Defence Secretary James Mattis announced last month that military chiefs had six months to determine if allowing transgender people in the armed services would affect the "readiness or lethality" of the force.
A debate is also raging over the Obama-era practice of requiring the Pentagon to pay for medical treatment related to gender transition in the military, reports the New York Times.
Republican Congresswomen Vicky Hartzler proposed an amendment to a $700bn (£533bn) budget bill that would bar the Pentagon from spending money on transition surgery or related hormone therapy, similar to that given to WikiLeaks whistleblower Chelsea Manning.
Aaron Belkin, director of the Palm Centre think-tank which studies gender and sexuality in the military, said Trump's decision would force transgender troops to in effect live as gays and lesbians did under Don't Ask, Don't Tell, the US military's ban on openly gay and lesbian servicemen and women which was lifted in 2011, reports the BBC.
"It's not clear why the president would want to bring it back now for transgender troops, when all the evidence suggests that inclusive policy benefits the military and discrimination hurts the military," he said.
There is no official tally of transgender people serving in the military and estimates range from 2,450 to 15,000. NPR said it was "not immediately clear" how the ban would affect those already openly serving in the armed forces.
