Under President Donald Trump's new rules, set to go into effect on April 12 after the Supreme Court allowed them to go forward, only transgender troops serving in their birth gender will be able to stay in the military after that date, and they will be barred from taking hormones or getting transition surgery.

The move was a reversal from a policy enacted under Trump's Democratic predecessor Barack Obama, who allowed military recruits to serve in their preferred sex rather than just the one they were assigned at birth.

The latest action was swiftly condemned by Democrats and civil rights advocates.

"The Trump administration is determined to bring back 'don't ask, don't tell,' a policy that forced service members to choose between serving their country and telling the truth about who they were," said Aaron Belkin, director of the Palm Center, an independent research institute.

Trump had announced the new ban by tweet in July 2017, pointing to the "tremendous medical costs and disruption that transgender in the military would entail.

After a flurry of legal battles, the Pentagon's policy is not a complete ban. Transgender troops have until next month's deadline to receive hormonal treatment or surgery, and service members who have already joined the military in their preferred gender will be able to continue to serve in that gender.

After April 12, however, no one who has transitioned to another gender or been diagnosed with "gender dysphoria" will be able to enlist -- and currently enlisted troops will thereafter have to serve in their birth gender and renounce transition treatment.

Around 9,000 people who self-identify as transgender individuals currently serve in the armed forces, out of 1.3 million in the active duty troops population, a Pentagon official said on condition of anonymity.

Around 1,000 say they have transitioned to another gender or would like to do so.

"Persons with a history of gender dysphoria -- a serious medical condition -- and who have undergone certain medical treatment for gender dysphoria, such as cross-sex hormone therapy or sex reassignment surgery, or are unwilling or unable to meet the standards associated with their biological sex, could adversely impact unit readiness and combat effectiveness," a Pentagon statement read.

"For this reason, such persons are presumptively disqualified for service without a waiver."