Story highlights Johnson's veiled warning to incoming President Donald Trump could be useful in upcoming court cases

Obama in 2012 signed an order that stops deporting some young undocumented immigrants

Washington (CNN) Department of Homeland Secretary Jeh Johnson is warning lawmakers that personal information collected by his agency for applications for Deferred Action for Childhood Arrival, or so-called Dreamers, should not be used in the future to deport them.

Johnson's veiled warning to incoming President Donald Trump and his administration could be useful in upcoming court cases if Trump revokes an executive order signed by President Barack Obama. That order, signed in 2012, stops deporting young undocumented immigrants who entered the United States as children so long as they meet certain requirements.

"Since DACA began, thousands of Dreamers have been able to enroll in colleges and universities, complete their education, start businesses that help improve our economy, and give back to our communities as teachers, medical professionals, engineers, and entrepreneurs -- all on the books," Johnson wrote in a letter. "We continue to benefit as a country from the contributions of those young people who have come forward and want nothing more than to contribute to our country and our shared future."

Johnson said it is department precedent to only use the details of people seeking protection from deportation only for that express purpose, citing how foreign victims of Hurricane Katrina or abused immigrants under the jurisdiction of the Violence Against Women Act were treated.

"We believe these representations made by the US government, upon which DACA applicants most assuredly relied, must continue to be honored," Johnson wrote.