Transgender troops can serve openly in the military effective immediately, Defense Secretary Ash Carter announced Thursday.

"The reality is we have transgender service members serving in uniform today," Carter said at a news briefing. "Americans who want to serve and can meet our standards should be afforded the opportunity to do so."

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“We have reason to be proud today of what this will mean for our military because it’s the right thing to do,” he added later.

Under the policy announced Thursday, troops currently in the military can no longer be discharged, separated or declined re-enlistment solely on the basis of gender identity.

Recruits, however, will have to wait until a one-year implementation period ends to enlist.

“Deliberate and thoughtful implementation will be key,” Carter said.

In 90 days, the Pentagon will issue a guidebook for leaders to follow, and troops will have access to medically necessary care, including surgery.

Commanders will have a say in the timing of medical care, as they do for any troop’s medical issues, so as not to conflict with deployments and other situations unique to the military, Carter said.

Troops will also be able to change their gender in the Pentagon’s personnel system at the 90-day point, Carter said.

When transgender recruits are allowed to enlist, they will have to be certified by a doctor has having been clinically stable for 18 months, Carter said.

Estimates on the number of transgender troops in the military range from 2,450, according to a 2016 RAND Corporation study, to 8,800, according to a 2014 UCLA Williams Institute study.

Under the policy, the military will consider gender transitions as beginning when a doctor diagnoses that medical care is necessary. At that point, troops will be allowed to follow the standards of their gender identity and use bathrooms, showers and other facilities of their gender identity.

The services have nine months to create training on the new policy.

Advocated of lifting the ban hailed Thursday’s announcement.

“Words cannot express how much this announcement means to so many of our transgender service members and their families — brave men and women who have proudly served our nation in silence for far too long," Ashley Broadway-Mack, president of the American Military Partner Association, said in a written statement. "We are incredibly grateful to Secretary Carter for bringing this promise to fruition.”

Meanwhile, Rep. Mac Thornberry (R-Texas), chairman of the House Armed Services Committee, slammed the announcement, saying key questions have yet to be answered.

“This is the latest example of the Pentagon and the president prioritizing politics over policy,” he said in a written statement. “The administration seems unwilling or unable to assure the Congress and the American people that transgender individuals will meet these individual readiness requirements at a time when our Armed Forces are deployed around the world."

- Updated at 2:38 p.m.