WASHINGTON — The Trump administration’s latest effort at banning transgender individuals from serving in the military amounts to what legal experts said is essentially a reprise of an all-too-familiar directive at the Pentagon: the 1994 “don’t ask, don’t tell” policy.

A slew of medical and legal professionals have already lined up against the newest White House order, issued late Friday night, which experts said may survive only if it is backed by the Supreme Court.

That order is based on recommendations outlined by Defense Secretary Jim Mattis in a Feb. 22 memo on the circumstances under which transgender people would be allowed to join or remain in the military. The memo by Mr. Mattis disqualified all people “who require or have undergone gender transition.”

But he also recommended that, going forward, “transgender persons without a history or diagnosis of gender dysphoria, who are otherwise qualified for service, may serve, like all service members, in their biological sex.”

The memo said that transgender troops who would be grandfathered into the system — under an Obama administration policy that allowed them to serve — could be discharged from the military if they continue to challenge the recommendations in courts.

Supporters of transgender rights said the recommendations echoed “don’t ask, don’t tell,” which was repealed in 2010 after 17 years of requiring gay men, lesbians and bisexuals in the military to keep their sexuality a secret.

“What they’ve basically done is said that if you have gender dysphoria, you’re out, with the exception of being grandfathered in, but if that’s used against them in court, you’re definitely out,” said Brad Carson, the former Defense Department acting under secretary for personnel and readiness during the Obama administration.

“They’re also basically saying that if you just keep quiet, we’ll leave you alone,” Mr. Carson said. He suggested that inherent contradiction is where a legal defense of the new policy could fall apart in court.