The White House memo also directs the Pentagon to deny admittance to transgender individuals and to stop spending on medical treatment regimens for those currently serving, according to U.S. officials familiar with the document.

The 2½-page memo gives Mr. Mattis six months to prepare to fully implement the new ban, according to these officials.

Mr. Mattis under the new policy is expected to consider “deployability”—the ability to serve in a war zone, participate in exercises or live for months on a ship—as the primary legal means to decide whether to separate service members from the military, the officials said.

The policy was announced by President Donald Trump in a series of tweets on July 26, which effectively reinstated a ban on open transgender service that had been lifted the year before, under former President Barack Obama, in a move that also provided for military medical care for the condition known as gender dysphoria.

Gender dysphoria is a condition that many professional associations have said requires medical treatment. For instance, gender dysphoria is recognized by the American Psychiatric Association as a medical issue. When diagnosed by a medical professional, transition therapy and reassignment surgery is considered by some insurers and states as a medically necessary treatment.

Employing the criteria of deployability to remove service members is bound to be greeted with deep opposition.