Video report by ITV News Correspondent Emma Murphy
Donald Trump has announced transgender people will not be accepted or allowed in the US military "in any capacity".
In a series of tweets, the US president said the decision was based on talks with generals and military experts.
Mr Trump's announcement on Twitter did not say what would happen to transgender people already in the military.
He said: "After consultation with my Generals and military experts, please be advised that the United States Government will not accept or allow transgender individuals to serve in any capacity in the U.S. Military.
"Our 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. Thank you."
This is a reversal of policy changes initiated under President Barack Obama, whose administration had set 1 July 2017 as the deadline for beginning to enlist openly transgender troops.
The policy had looked on shaky ground when, earlier this month, US Defence Secretary James Mattis, approved a six-month delay to the plan.
There are as many as 250 service members in the process of transitioning to their preferred genders or who have been approved to formally change gender within the Pentagon's personnel system, according to several defence officials.
A Rand Corporation study estimated that there are between 2,500 and 7,000 transgender service members on active duty and an additional 1,500 to 4,000 in the reserves.
Mr Trump's decision drew swift reaction from critics, while some conservative organisations and politicians hailed the decision.
Caitlyn Jenner criticised the president for breaking a promise to fight for the LGBT community.
The 67-year-old, who voted for Trump and is a Republican Party supporter, tweeted: "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?"
Star Wars actor Mark Hamill issued a message of support to the transgender people currently serving in the US military.
The hashtag #TransRightsAreHumanRights trended on Twitter, with many using it to attack the president's announcement.
Family Research Council president Tony Perkins applauded Mr Trump for "keeping his promise to return to military priorities - and not continue the social experimentation of the Obama era that has crippled our nation's military".
Iowa representative Steve King said Mr Trump's supporters "will be happy to hear it. We don't' need to be experimenting with the military. Plus there's no reason to take on that kind of financial burden".
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 October 1, transgender troops have been able to receive medical care and start formally changing their gender identifications in the Pentagon's personnel system.
Mr Carter also gave the services until July 1 to develop policies to allow people already identifying as transgender to newly join the military. Mr Mattis announced earlier this month that he was giving military chiefs another six months to conduct a review to determine if allowing transgender individuals to enlist in the armed services would affect the "readiness or lethality" of the force.
Key concerns include whether currently enlisted troops have had medical or other issues that cause delays or problems with their ability to deploy or meet physical or other standards for their jobs.
Military leaders also wanted to review how transgender troops are treated, if they are discriminated against or if they have had disciplinary problems, defence officials have said.