WASHINGTON — President Trump is preparing to give the Defense Department authority to expel transgender people from the military in an upcoming order, barring the Pentagon from recruiting transgender troops, and cutting off payment for sexual reassignment surgery and other medical treatments for those already serving.

A White House memo that is expected to be sent to the Pentagon in the coming days gives Jim Mattis, the secretary of defense, six months to enforce the transgender ban that Trump announced abruptly last month in a series of tweets. The directive was confirmed Wednesday by a person familiar with its contents but who was not authorized to discuss its details and spoke on the condition of anonymity.

The authority has not been finalized. Once it is approved, it would allow Mattis to force out transgender service members by setting a legal standard of whether they would be able to deploy to war zones or for other lengthy military missions.

The president’s order caught senior military officials by surprise and short-circuited the customary interagency process that attends such sweeping decisions. At the time, as senior military officials scrambled to determine how to carry out the order, White House officials said they would work with the Pentagon to devise a policy to fit Trump’s tweets.

Sarah Huckabee Sanders, the White House press secretary, declined late Wednesday to comment on any forthcoming guidance, saying the White House had no announcement on the matter. The memo was first reported by The Wall Street Journal.

Advocates of allowing transgender people to serve openly said the guidance imposed an unacceptable double standard.

“It is unconscionable that the commander in chief would take aim at his own, loyally serving troops for political reasons at a time when the military needs to focus on real threats,’’ said Aaron Belkin, the director of the Palm Center, a research institute that had worked with the military to devise its policy on admitting transgender service members.

“Imposing one set of standards for transgender troops and another set of standards for everyone else is a recipe for disruption, distraction, and waste,’’ Belkin said.

In announcing the ban in July, Trump declared that US forces could not afford the “tremendous medical costs and disruption’’ of transgender service members.

The president said he had consulted generals and military specialists, but Mattis was given only a day’s notice about Trump’s decision.

The guidance — basing expulsion on a service member’s ability to serve — appears to be an attempt to reconcile Trump’s call for a blanket ban with concerns about whether the defense secretary should dismiss transgender forces who are currently in the ranks.

Trump’s decision was roundly denounced by members of both parties, many of whom argued that people willing and able to fight for their country should be welcomed into the military.

“This is NOT how you keep America safe,’’ Representative Nancy Pelosi of California, the minority leader, said in a Twitter post. “Period. #ProtectTransTroops.’’

The ban reverses a year-old policy crafted by the Obama administration that allowed transgender people to serve openly in the military.

That policy affects only a small number of the approximately 1.3 million active-duty members of the military. Some 2,000 to 11,000 active-duty and reserves troops are transgender, according to a 2016 RAND Corp. study commissioned by the Pentagon, although estimates of the number of transgender service members have varied widely and are sometimes as high as 15,000.

The issue became a flash point for social conservatives who argued that transgender people had no place in the military.

Some Republican lawmakers threatened to refuse to fund the military without a prohibition on using federal money to pay for transition surgery or related hormone therapy.

“As transgender service members, we are and have always been soldiers, sailors, airmen, Marines, and Coast Guardsmen first,’’ said Blake Dremann, the president of Sparta, an LGBT military group with 500 active-duty members. “We serve our country honorably, in good faith.’’