The U.S. military is returning to the era of "Don't Ask, Don't Tell" policies, said troops and LGBT+ groups, as new rules that will ban most openly transgender people from serving came into effect on Friday.
They said the new policy would force trans military members to choose between transition and their job and result in increased stigma and mental health issues.
"With the implementation of this transgender military ban, our nation is once again shamefully forcing brave American heroes to hide who they are in order to serve," said Ashley Broadway-Mack, president of the American Military Partner Association.
"By dragging us backward into the dark days of 'Don't Ask, Don't Tell,' the Trump-Pence administration is inflicting tremendous harm on our service members, their families, and the military as a whole.”
The Department of Defense said not all trans military members would be affected.
"The military provides all necessary medical care to protect the health of our service members, including those who are in the process of being separated," a spokeswoman added.
More than 10,000 transgender people may be serving in the U.S. military across active service and reserves, according to a 2016 study by the RAND Corporation, a think tank.
But they have rarely been able to be open about their identity or transition while serving.
While the official "Don't Ask, Don't Tell" policy that prevented sexual minorities from serving openly was brought to an end in 2011, it was only in 2016 that former President Barack Obama lifted restrictions barring trans people.
That policy was reversed by the current commander-in-chief, President Donald Trump, on the basis that trans people cause "tremendous medical costs and disruption" to the military.
The new policy bars those who have transitioned or are openly transgender from enlisting after Friday, while troops who come out as trans while serving after that day will be discharged.
But it allows members of the armed forces who were diagnosed with gender dysphoria during the Obama policy to remain in the military and serve according to their gender identity.
Kara Corcoran, a captain in the U.S. Army, disclosed that she was trans last year, under pressure from the incoming rules that would have forced her to stay in the closet indefinitely.
"I cannot explain to you how much more mentally sound and even more physically fit I am today than I was a year ago," she said of the decision.
She feared that many other troops who were not yet ready to come out would suffer, saying she had personally recently spoken to a colleague who had decided to keep his identity a secret.
"He's like 'I'll hold out and hopefully they will change the policy back,' and it's hurtful because I know that he will eventually get to a point where he can't suppress it like I did for so many years," she said.
Studies suggest there are likely to be thousands of trans troops who have not disclosed their identity and will be forced to choose between their career and living openly, said Blake Dremann, of the SPART*A organization for trans military members.
The rules will have an impact on the mental health of trans troops, said Jillian Scheer, a postdoctoral research fellow at Yale University.
"The president stating these messages reinforces this notion that trans people are different, and therefore should be stigmatized, that they are not fit to serve," she said.
The policy was likely to cause increased anxiety, depression and suicidal feelings among trans service members, she said, and could also lead to a spike in stigma and abuse of trans people in wider society.
LGBT+ organizations are fighting the new rules in the courts, arguing they violate trans Americans' constitutional rights including equal protection under the law, due process and freedom of expression.
The courts have allowed the administration to put the policy into effect, but they have not yet ruled on the wider issues of whether it is legal.
"When this policy is examined closely there is no justification that could possibly explain what the government is doing," said Peter Renn, of law firm Lambda Legal, the lead attorney in one of the cases fighting against the new rules.
"We are positive that when we put forward all the facts... the courts will see it as baseless, irrational discrimination.”
He said, however, that it could be months before the court process is completed and a final judgment issued.
Meanwhile, trans military groups said they would continue to support those forced to hide their gender identity as a result of the changes.
"It's living as two different people," said Dremann.
"That type of thing is untenable - a lot of people can do it for a certain amount of time but eventually they are going to have to transition.
"And no one should have to choose between getting care that they need and being able to serve their country.”