Story highlights Trump announced in late July via Twitter that transgender individuals would no longer be allowed to enlist or serve in the military

US defense officials have indicated that they were caught off guard by the policy change

Washington (CNN) Five active duty transgender service members filed the first lawsuit Wednesday against President Donald Trump's directive -- expressed on Twitter -- to prohibit transgender individuals from serving in the armed forces.

The service members, who are not named, all say they have relied on the Defense Department's current policy permitting open service by transgender service members and argue Trump's ban, which may result in early termination or failure to renew their contracts, is unconstitutional.

The President's three-tweet plan to stop transgender individuals from serving in the military has yet to be formally implemented, but attorneys for the service members have asked a federal court in Washington to block it immediately.

Trump's directive has "already resulted in immediate, concrete injury to Plaintiffs by unsettling and destabilizing plaintiffs' reasonable expectation of continued service," they argue in a court filing, adding that the White House has turned Trump's tweets into "official guidance, approved by the White House counsel's office, to be communicated to the Department of Defense."

The Defense Department is still awaiting formal guidance on implementation from the White House, spokesman Lt. Col. Paul Haverstick told CNN.

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