President Trump signed a presidential memo Friday instructing the Defense Department to stop accepting transgender people who want to enlist in the military.

The memo details Trump's previous Twitter announcement last month that he would reinstate a ban on transgender people serving in the U.S. military, and officially requests the Pentagon begin implementing the ban.

Trump’s memo bars transgender people from enlisting, but instructs Secretary of Defense James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE to further explore how to handle transgender people currently serving in the armed forces.

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The memo also orders the Pentagon to stop paying for gender reassignment surgeries, except in cases that are already in progress to “protect the health of an individual.”

It also requests that the Pentagon develop an implementation plan for the ban by Feb. 21 2018, to be put in place on March 23, 2018.

On a call with reporters prior to the memo’s release, a White House official said Trump “spoke passionately” about the freedoms of the LGBT community, but based the policy on a “series of national security considerations,” according to reports.

Pentagon spokeswoman Dana White said in a statement that “the Department of Defense has received formal guidance from the White House in reference to transgender personnel serving in the military.”

More information on the guidance will be provided early next week, Pentagon spokesman Lt. Col Paul Haverstick told The Hill.

The news follows Trump’s tweets last month announcing his abrupt decision to reinstate a ban on transgender people serving in the U.S. military.

Trump wrote on Twitter that transgender troops could no longer serve “in any capacity."

Among his reasons, Trump wrote that the Pentagon “cannot be burdened with the tremendous medical costs and disruption” brought by transgender service members.

The new guidance reverses an Obama administration policy implemented in July 2016 that allows transgender people to serve openly in the military.

Trump’s memo frames Obama’s policy — which was crafted after a working group spent a year to hammer out the details and implementation — as dismantling the Pentagon’s “established framework.”

It also claims the previous administration did not “identify a sufficient basis to conclude that terminating the Departments' longstanding policy and practice would not hinder military effectiveness and lethality, disrupt unit cohesion, or tax military resources,” therefore requiring further study.

Former Defense Secretary Ash Carter in June 2016 ordered the Pentagon to spend a year to study how to allow transgender individuals to join the military.

Mattis in June extended the study through January 2018, but it was abruptly thrown off after Trump announced the ban in July.

The presidential memo immediately drew outrage from LBGT advocacy groups.

The GLBTQ Legal Advocates and Defenders (GLAD) and the National Center for Lesbian Rights (NCLR) – which jointly filed a federal suit on Aug. 9 on behalf of five transgender troops contesting the ban – in a Friday statement called the memo a “senseless and unprecedented attack on dedicated service members who have played by the rules.”

“Our military already has standards and systems in place to ensure our troops are qualified and fit to serve,” said Shannon Minter, NCLR Legal Director. “Trump's ban is about politics, not military policy, and it will make our country less secure.”

GLAD Transgender Rights Project Director Jennifer Levi said the policy is “a shameful slap in the face to people who put their lives on the line everyday to defend our country.”

”Our military is strongest when all people who are fit to serve have the opportunity to do so. This unprecedented policy amounts to a purge of qualified, contributing troops, and will serve only to undermine unit cohesion and weaken military readiness,” Levi said.

The two groups also plan to file a motion in D.C. district court.

The transgender ban policy already faces intense opposition from top Republican senators, dozens of retired generals and admirals and advocacy groups.

This story was updated at 7:38 p.m.