The White House has announced orders to formally ban transgender people from serving in the military, following up on Donald Trump’s controversial policy pledge that sparked widespread backlash last year from civil rights groups and US defense chiefs.

Despite opposition from top military officials and previous rulings against the ban, which LGBT rights groups have challenged in court, a memo from the secretary of defense released late on Friday night said trans people are “disqualified from military service except under certain limited circumstances”.

The memo did not elaborate on possible exceptions, but said the secretary of defense and secretary of homeland security “may exercise their authority to implement any appropriate policies concerning military service by transgender individuals”.

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The announcement claimed that the policy was “developed through extensive study by senior uniformed and civilian leaders” and said that based on the advice of “experts”, the administration has concluded that the “accession or retention” of trans people “presents considerable risk to military effectiveness and lethality”.

When Trump originally announced the ban on Twitter, he claimed that the military “cannot be burdened with the tremendous medical costs and disruption” of trans service members, despite the fact that studies have found that the ban would have negative impacts on the military.

Critics have argued that the discriminatory policy will cause significant disruptions in the military, force trans members to hide their identities, deprive the military of talented people and reinforce harmful and inaccurate stereotypes about trans people. The president’s orders have reversed Barack Obama’s policy in 2016, which ended a longstanding rule barring trans people from serving openly in the military.

“What the White House has released tonight is transphobia masquerading as policy. This policy is not based on an evaluation of new evidence,” Joshua Block, a senior staff attorney at the American Civil Liberties Union’s LGBT and HIV Project, said in a statement on Friday night. “It is reverse-engineered for the sole purpose of carrying out President Trump’s reckless and unconstitutional ban, undermining the ability of transgender service members to serve openly and military readiness as a whole.”

Facebook Twitter Pinterest Thousands of New Yorkers took the streets of in opposition of the proposed policy change in July 2017. Photograph: Michael N/Pacific/BarcroftImages

Trump’s directive was already blocked by federal courts in four separate cases, including an ACLU lawsuit, the group noted.

“The policy effectively coerces transgender people who wish to serve into choosing between their humanity and their country,” Block said. “Transgender people in our military deserve more from their government than a myth-driven proclamation about their unfitness for full civic inclusion.”

Sarah McBride, the national press secretary with the Human Rights Campaign, the largest LGBT rights group in the US, also condemned the policy in a statement, saying: “Releasing this news under cover of darkness late on a Friday night means thousands of transgender troops will wake up tomorrow with their lives turned upside down. This couldn’t be more cowardly or wrong.”

Numerous Republican senators, including John McCain, Susan Collins and Orrin Hatch, have criticized the policy, and more than 50 retired generals and admirals signed a letter saying the ban would degrade military readiness.

The House Democratic leader, Nancy Pelosi, denounced the new announcement in a statement, saying: “This latest memorandum is the same cowardly, disgusting ban the president announced last summer. No one with the strength and bravery to serve in the US military should be turned away because of who they are.”

Trump has also rescinded Obama-era protections for trans students, and vice-president Mike Pence has a long history of backing anti-LGBT policies.