President Donald Trump has announced that transgender people will be barred from serving in the US military “in any capacity,” due to the “tremendous medical costs and disruption” they impose.

President Trump made the pronouncement on Twitter, reversing a groundbreaking decision by President Barack Obama last year to open the armed services to transgender troops.

The President on Wednesday night tweeted that he made his decision after consulting with “generals and military experts,” without naming them.

He said the US military “must be focused on decisive and overwhelming victory and cannot be burdened with the tremendous medical costs and disruption that transgender in the military would entail”.

No mention was made of the many of thousands of transgender troops already serving in the military.

There is thought to be at least 4000, and as many as 10,000, transgender troops serving in the US armed services.

Whistle-blower Chelsea Manning is perhaps the best-known transgendered person to have served in the US military.

After consultation with my Generals and military experts, please be advised that the United States Government will not accept or allow……

— Donald J. Trump (@realDonaldTrump) July 26, 2017

….Transgender individuals to serve in any capacity in the U.S. Military. Our military must be focused on decisive and overwhelming…..

— Donald J. Trump (@realDonaldTrump) July 26, 2017

….victory and cannot be burdened with the tremendous medical costs and disruption that transgender in the military would entail. Thank you

— Donald J. Trump (@realDonaldTrump) July 26, 2017

White House spokeswoman Sarah Huckabee Sanders told a media briefing Thursday morning (AEST) that President’s Trump’s announcement was a military decision, and “not meant to be anything more than that”.

“Based on consultation that he’s had with his national security team, he came to the conclusion that it erodes military readiness and unit cohesion,” she said.

Ms Sanders could not answer what would happen to currently serving transgender troops, saying only that the White House and the Department of Defense would work together “as implementation takes place and is done so lawfully”.

In a brief written statement, Pentagon spokesman, Navy Captain Jeff Davis said the Pentagon was working with the White House to “address” what he called “the new guidance” from the President.

Transgender service members have been able to serve openly in the military since last year, when former Defence Secretary Ash Carter ended the ban.

Since last October 1, they have been able to receive medical care and start formally changing their gender identifications in the Pentagon’s personnel system.

Already, there are as many as 250 service members in the process of transitioning to their preferred genders.

A 2016 study commissioned by the US Defense Department found that allowing transgender people to openly serve in the military would have a “minimal impact” on readiness and health care costs, largely because there are so few in the military’s 1.3 million-member force.

President Trump’s decision is in stark contrast to his election pledge last year to “fight for the LGBT community.

Angry response

Senator Tammy Duckworth of Illinois, a double amputee veteran of the Iraq War, said that when her Black Hawk helicopter was shot down, she didn’t care “if the American troops risking their lives to help save me were gay, straight, transgender or anything else. All that mattered was they didn’t leave me behind.”

Matt Thorn, executive director of OutServe-SLDN, which represents the LGBT population in the military, said thousands have been serving in the US armed forces without causing any issues.

“It’s an absolute absurdity and another overstep,” Thorn said. He threatened legal action if Wednesday’s decision is not reversed.

Senator John McCain, a Vietnam veteran who chairs the Senate Armed Services Committee, said the tweet was “another example of why major policy announcements should not be made via Twitter”.

“Any American who meets current medical and readiness standards should be allowed to continue serving,” Senator McCain said.

“There is no reason to force service members who are able to fight, train and deploy to leave the military-regardless of their gender identity.”

Staunch Republican Caitlyn Jenner, who won a decathlon Olympic gold medal in 1976 as Bruce Jenner, described the decision as breach of Mr Trump’s 2016 election promise to “fight for the LGBT community.”

Thank you to the LGBT community! I will fight for you while Hillary brings in more people that will threaten your freedoms and beliefs.

— Donald J. Trump (@realDonaldTrump) June 14, 2016

“There are 15,000 patriotic transgender Americans in the US military fighting for all of us. What happened to your promise to fight for them?” she asked.

– With agencies