A transgender former US Navy SEAL has called "bulls---" on President Donald Trump's reasoning behind the ban on military service for some transgender people.

Kristin Beck, a decorated veteran of the SEALs' most elite unit, said the Department of Justice and the Pentagon's assertion that the lethality and effectiveness of the military would decrease with transgender service was false.

Beck previously called out Trump on the ban, telling the president to say it to her face.

President Donald Trump's ban on military service for some transgender people got the go-ahead from the Supreme Court on Tuesday, but a transgender former US Navy SEAL has called "bulls---."

Both the Pentagon and the Department of Justice welcomed the high court's move to lift an injunction on Trump's transgender military ban, which will allow him to enforce his policy barring certain transgender troops from joining or staying in the military.

Read more: The Supreme Court is allowing Trump to implement the transgender military ban

The US policy under question bars from military service, with some exceptions, people who have gender dysphoria — a disconnect between a person's gender identity and their sex assigned at birth — or are seeking to transition or have transitioned, people whose medical diagnosis would require a transition during service, and people who can't demonstrate "stability" in their assigned sex for at least the three years before applying to serve. The Pentagon has claimed that gender dysphoria impairs functionality.

While the Pentagon insisted to Business Insider the above criteria did not equate to a ban on transgender military service, it's unclear whether most transgender people could meet those requirements.

"Due to lower courts issuing nationwide injunctions, our military had been forced to maintain a prior policy that poses a risk to military effectiveness and lethality for over a year," the Department of Justice said in a statement seen by Business Insider.

Former Defense Secretary Jim Mattis wrote last February that the inclusion of such people in the military would decrease the "readiness, lethality, and effectiveness of our military."

The US's top warfighters beg to differ

Beck in Panama. Pinterest/Kristin Beck

Beck, who served for 20 years in the Navy with SEAL Teams 1, 5, and, eventually, the elite 6, called the Trump administration's position on transgender military service "bulls---."

Beck told Business Insider on Tuesday that the Pentagon had been "intentionally confusing" on the issue of trans service and that it failed to even define the concepts of lethality and effectiveness.

"I was a Navy SEAL for 20 years — I was pretty lethal," she said.

Read more: Meet Kristin Beck, the transgender Navy SEAL hero fighting Trump’s proposed trans ban

Beck deployed 13 times, including stints in Bosnia, Iraq, and Afghanistan, earning the Bronze Star award for valor and the Purple Heart for wounds suffered in combat.

The "idea transgender effects 'Lethality and effectiveness' which is total BULL SHIT," she tweeted on Tuesday, describing the Supreme Court's decision as "wrong."

She added: "18 other Nations ALLOW #Trans service members... zero issues!"

Beck has previously had tough words for Trump, responding to his initial announcement of the ban in July 2017 — which shocked the Pentagon — with an open challenge.

"Let's meet face-to-face and you tell me I'm not worthy," Beck told Business Insider. "Transgender doesn't matter. Do your service."

While Beck's unique point of view found no home in the Trump administration or with the Pentagon officially, the top leaders of the US Army, Navy, Air Force, and Marine Corps all said in April that they found no issues with transgender people serving.