The presidential memorandum signed by Trump will also prevent the military from providing medical treatment for sex reassignment treatments.
Trump is reversing an Obama administration policy that was the latest step in the advancement of rights for lesbian, gay, bisexual and transgender people in the U.S. military in recent years. But the White House said Trump believes his predecessor failed to provide sufficient basis to determine whether terminating the Department of Defense's long-standing policy on transgender individuals would "hinder military effectiveness and lethality, disrupt unit cohesion, or tax military resources."
The memorandum signed by the president states that "further study" is necessary to ensure that the policy change would not have negative effects on military readiness.
In June 2016, President Barack Obama allowed transgender service members to serve openly in the military and receive related medical treatment while ordering the Pentagon to determine a policy for allowing transgender people to join the armed forces within a year.
But Defense Secretary Jim Mattis earlier this year delayed the entry of transgender troops until January 2018, citing the need to further evaluate the impact on the military's "readiness and cohesion."
The memorandum comes weeks after Trump abruptly announced the ban in a series of tweets on July 26. It was not immediately implemented by the military, which said it would wait for further guidance from the White House.
"After consultation with my Generals and military experts, please be advised that the United States Government will not accept or allow transgender individuals to serve in any capacity in the U.S. Military," Trump wrote in July 26 tweet. "Our military must be focused on decisive and overwhelming victory and cannot be burdened with the tremendous medical costs and disruption that transgender in the military would entail."
Trump's initial announcement was greeted with criticism from Democrats and Republicans, many of whom said it was discriminatory toward transgender people willing to volunteer to serve their country.
"Any American who meets current medical and readiness standards should be allowed to continue serving," said Sen. John McCain, R-Ariz., in a statement issued after Trump's Twitter announcement of the ban. "There is no reason to force service members who are able to fight, train and deploy to leave the military - regardless of their gender identity."
The memo released Friday gives the Pentagon six months to determine how to implement Trump's directive as it relates to transgender people currently serving. Until that determination is made, no action will be taken against any individuals, the memo states. That plan will be implemented by March 23, 2018.
A senior administration official defended the president's actions in a call with reporters on Friday, saying the decision was made based on national security concerns.
"The basis for this directive is very much our national security needs and things like military readiness, effectiveness, lethality, unit cohesion, etc," said the official, who spoke on a condition of anonymity to discuss the details of the policy. "He's going to continue to ensure that the rights of the LGBT community as well as all Americans are protected."
Democrats and civil rights groups heavily criticized the announcement, calling it a step backward.
"The Commander-in-Chief is ordering the Pentagon to hurt and humiliate thousands of Americans who serve in our military with strength and courage," House Democratic Leader Nancy Pelosi, D-Calif. "Prejudice, not the national defense, is behind President Trump's cruel decision to kick transgender troops out of the military."
The prohibition on funding sex-reassignment surgeries will also go into effect on March 23, 2018, with an exception for individuals currently undergoing the procedures.
Trump's decision thrust his administration into a contentious debate about rapidly changing norms around sexual orientation and gender identity. The ban on transgender servicemembers was the last such prohibition to be removed after the "Don't Ask, Don't Tell," policy toward openly gay and lesbian servicemembers was repealed in 2010.
Trump's decision was made, in part, because of strenuous lobbying from conservative members of Congress who had threatened to hold up a spending bill with money allocated for a border wall - a key policy priority for the president - if the military was allowed to pay for sex-reassignment surgeries to transgender individuals.
While Trump addressed that issue, he also made far more sweeping changes to the military's policy toward transgender individuals, surprising many in his own party and even advocates who opposed allowing transgender people to serve.
Trump's announcement was followed by no action for weeks and his short-circuiting of the policymaking process on this issue caught military officials by surprise.
Pentagon spokeswoman Dana White acknowledged on Friday that the Department of Defense had received Trump's memorandum.
"More information will be forth coming," White said.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)