A former chairman of the Joint Chiefs of Staff has thrown his support behind a lawsuit against President Trump's ban on transgender troops.

“The military’s prior considered judgment on this matter should not be disregarded, and we should not breach the faith of service members who defend our freedoms, including those who are transgender,” retired Adm. Michael Mullen wrote in a declaration in support of Lambda Legal and OutServe-SLDN’s lawsuit.

Mullen was the highest-ranking officer in the military from 2007 to 2011. In that role, he oversaw the end of "don’t ask, don’t tell,” the repealed law that banned gay and lesbian soldiers from serving openly.

“Just as gay and lesbian soldiers should not have to lie about who they are to serve, nor should transgender soldiers,” he wrote in his declaration. “When I led our armed forces under DADT, I saw firsthand the harm to readiness and morale when we fail to treat all service members according to the same standards. There are thousands of transgender Americans currently serving, and there is no reason to single them out to exclude them or deny them the medical care that they require.”

In July, Trump tweeted that he would ban transgender people from serving in the military in any capacity.

He made good on the tweets in August, signing a presidential memo that prohibits the military from enlisting transgender people and from using funds to pay for gender transition-related surgery. The memo also gave Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE six months to determine what to do with currently serving transgender troops.

Days later, Mattis said he would establish a panel of experts to determine how to implement Trump’s order, but currently serving transgender troops would be allowed to continue serving while the study is ongoing.

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OutServe and Lambda Legal are representing six current troops, three people who wish to enlist, the Human Rights Campaign, Seattle-based Gender Justice League and the American Military Partner Association.

Their lawsuit is one of four against the ban. The others are from the National Center for Lesbian Rights (NCLR) and GLBTQ Legal Advocates & Defenders (GLAD), the American Civil Liberties Union (ACLU) and Equality California.

The NCLR and GLAD and the ACLU have asked for a preliminary injunction in their lawsuits.

In addition to getting Mullen’s support, Outserve and Lambda Legal also filed a motion to immediately halt the ban with a preliminary injunction, arguing the policy is already doing harm.

“It is unacceptable to destroy the careers of patriotic and courageous members of the U.S. military,” Peter Perkowski, legal director for OutServe, said in a statement. “This ban must be stopped dead in its tracks before it goes any further so that these brave men and women can focus on their real jobs — protecting and serving the country they love.”

Phillip Stephens, a five-year member of the Navy who is one of the troops represented in the suit, added that Trump tweets “pulled the rug out from under” transgender troops.

“It is impossible to overstate how important it was when the Pentagon lifted the ban on open service, when I and other transgender service members were finally able to live and serve as our true and authentic selves,” Stephens said in a statement.

“To read those tweets, to have the rug pulled out from under us, to be branded unfit to serve was devastating, not just for me, but really for the U.S. military and military readiness as a whole.”