A month after he announced a ban on transgender people serving in the military via Twitter, Donald Trump has issued a memo on how the decision is to be implemented.

The two-and-a-half-page note says Defense Secretary Jim Mattis must consider a service member's 'deployability' when deciding whether to eject them from the military.

That means that if they are unable to serve in a war zone, take part in training or serve on a ship for months, they must go, the Wall Street Journal reported.

It has not been spelled out exactly how the criteria will be assessed, however the Trump administration seems to be implying that it does not believe transgender service people are deployable.

The policy gives the Pentagon six months to oust 'non-deployable' transgender service members, bans new transgender hires and orders the Pentagon to stop paying for trans service members' medical treatments.

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Plan in action: Donald Trump's ban on transgender military members - which he announced in July - has now gone into action after the White House revealed how it was to be implemented

Kicking out: Defense Sec. Jim Mattis (right) must remove anyone who is 'non-deployable' - which includes transgender service members, according to the new guidelines

Plans: Until now the Pentagon had no rules by which to manage or justify the removal of any transgender service members, who were told they could come out in 2016 by Barack Obama

It has been criticized by transgender representatives.

'Transgender people are just as deployable as other service members,' said Sue Fulton, the former president of Sparta, a military organization for LGBT people that advocates for open service.

'Other service members may undergo procedures when they are at home base, just as other service members schedule shoulder surgery or gall bladder surgery,' added Fulton, who achieved the rank of captain in the army, and is not transgender.

She said that there are no 'ongoing treatments' that would render transgender soldiers, sailors and pilots non-deployable.

'Thus there's no difference between the deployability of transgender service members' and that of others, she said.

A Rand Corp study commissioned last year estimated that there were between 1,320 and 6,000 transgender people openly serving in the military.

Advocacy groups put the figure for those on active duty at 7,000 and total figures across all areas of the military at 11,000.

The Rand Corp survey noted that not all of them seek treatment - and those that do have treatment that would render them non-deployable are few in number.

Using surveys and private health insurance data, the study concluded that only 29 to 129 members in the military's active component would be rendered non-deployable by their planned treatment.

Opposed: Almost 60% of Americans are against the ban. Ex-Army captain Sue Fulton (right) said nothing about trangender people or their ongoing treatment makes them non-deployable

Standing up: Fulton, who campaigns for LGBT army rights but is not trans, says even if surgery is to be performed, it's just like having a gall bladder operation on a military base

The new memo is the first attempt by the government to provide guidance to the Pentagon on how to follow through on the rule Trump announced on July 26.

He tweeted: '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 US 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.'

The Rand Corp's survey concluded that it would cost between $2.4 million and $8.4 million a year to treat transgender service members - that's 0.04-0.13 per cent of 2014's $6.27 billion military budget.

It's also just 2.86-10 per cent of the $84 million spent by the US military on Viagra and similar sexual health supplements per year, according to Military Times.

Trump's announcement - which was opposed by 58 per cent of the country according to a July 28 Reuters/Ipsos poll - came after pressure from conservatives.

They were concerned by the July 1, 2017 opening date for transgender recruitment.

That was set by Barack Obama in 2016; at the same time, he immediately allowed transgender people already in the military to begin serving openly.

That has left some in the military's upper echelons concerned about how to justify implementing the ban - particularly as there are long-serving servicemen and women who announced they were transgender after Obama's order came through.

New rules: Barack Obama (pictured with transgender Staff Sgt Logan Ireland, right) lifted the DADT rules in 2016. A survey suggested just 29-129 of up to 6,000 transgender members would be rendered non-deployable by transitioning

Also outraged by the decision was Aaron Belkin, director of the Palm Center, an independent research institute that sponsors scholarship on policy issues.

'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,' Belkin said in a statement Wednesday.

'Former Chairmen of the Joint Chiefs of Staff Admiral Mike Mullen and General Martin Dempsey issued statements in support of our nation's transgender troops, and fifty-six retired Generals and Admirals concluded that banning transgender troops would "cause significant disruptions" and would "degrade readiness even more than the failed don't ask, don't tell policy."

'These distinguished military leaders are correct.'

He continued: 'Transgender troops have been serving openly in the US military for more than a year, and have been widely praised by Commanders.

'Eighteen foreign militaries allow transgender personnel to serve, and none have reported any compromise to readiness.

'All available research, including scholarly publications by the Rand Corporation as well as retired General and Flag Officers, has reached the same conclusion: inclusive policy for LGBT personnel promotes readiness.

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

Angry: Trump's decision angered protesters, and came after conservatives pressured him. They were concerned about the July 1, 2017 date Obama set for letting in transgender people

The same day as Trump's July tweets were made, transgender airman Staff Sgt. Logan Ireland told Air Force Times: 'I would like to see them try to kick me out of my military.'

'You are not going to deny me my right to serve my country when I am fully qualified and able and willing to give my life.'

Ireland - who has served in Afghanistan - first shared his story in 2015, three years after he began transitioning, and consulted with the Air Force in 2016 as it drafted guidance for admitting transgender people after Obama's announcement.

Another transgender man - corporal who did not wish to be named - told the site: 'I have never described myself as trans; I'm a motherf***ing Marine.'

'That's all that matters. Don't tarnish my title with your bigotry and fear of the unknown.'