Captain Simratpal Singh, 28 was in December last year granted a temporary religious accommodation to serve in the US army while maintaining his Sikh turban, unshorn hair and beard.
The accommodation, a rare exception, was scheduled to remain in effect until March 31 but in an unprecedented step backward, the US army recently ordered Singh to report on Tuesday for "extraordinary, non-standard additional testing as a precondition for remaining in the army", international law firm McDermott Will & Emery, representing Singh, said in a statement.
The firm, along with the Sikh coalition and the Becket Fund for Religious Liberty, filed the federal lawsuit against the department of defence on behalf of Singh, who has earned a Bronze Star for clearing explosives from roads in Afghanistan, and received numerous other military accolades in various positions.
Singh is scheduled to be sequestered for three days to undergo testing that no other soldier in the US army has ever been subjected to, including the soldiers permitted to maintain beards for medical reasons and previously-accommodated Sikh soldiers, the law firms said.
The New York Times said that the tests seek to determine if his helmet would fit over his long hair and if his gas mask could seal his face.
The lawsuit, which is the first of its kind on behalf of a Sikh officer, demands that the US military continue to accommodate Singh's Sikh turban, unshorn hair, and unshorn beard and "abandon its unfair and discriminatory" testing.
"For years we have worked to avoid litigation under the guiding belief that the US military would finally do the right thing," said Amandeep Sidhu, partner at McDermott Will and Emery.
"The US constitution and the Religious Freedom Restoration Act make it clear that Captain Singh has the right to practice his faith in the military and we are confident that the court will agree," said Sidhu.
Once the testing is enjoined, Singh seeks a further ruling directing the army to make his religious accommodation permanent.
"I have so much pride in my Sikh identity and service to my nation," Singh had said in December after receiving his temporary accommodation.
"To feel spiritually whole, while continuing my military career, has always been the dream," he had said.
Singh had enrolled in West Point in 2006 and after failed attempts to obtain an accommodation, he had to cut his hair and shave his beard in order to be able to serve in the army.
Singh, who graduated from West Point with honours in 2010, filed an accommodation request in October last year and was granted a temporary 30-day accommodation to serve while maintaining his Sikh articles of faith.
"Captain Singh is being subject to discriminatory testing that isn't required of any other soldiers, even those with medical or religious accommodations. The army cannot delay in providing him his statutory and constitutionally mandated right to an accommodation to serve as an observant Sikh in the army," said the Sikh Coalition's Legal Director, Harsimran Kaur.
Senior counsel at the Becket Fund for Religious Liberty Eric Baxter said the US army should be trying to get more soldiers like Singh instead of banning them from serving or punishing them for their beliefs.
"It's time for the Pentagon to stop playing games and start doing the right thing - for Captain Singh, for Sikh Americans, and for all Americans," Baxter added.
Last year, 27 retired US Generals called on the US department of Defence to eliminate the ban on observant Sikhs.
These generals join 105 Members of Congress, 15 US senators and 21 national interfaith and civil rights organisations, who have previously signed letters in support of American Sikhs' right to serve.
'A baby step forward for the Sikh community in US army'
Indo-Canadian Sikh named Canada's new defence minister
PHOTOS: Meet the first women soldiers from US army's toughest school
'I am a proud Sikh, proud American'
US Army gets first Sikh soldier since 1980s