President Donald Trump’s executive order denying U.S. citizenship to the children of parents living in the country illegally has faced the first of what will be many legal tests.

A federal judge Thursday blocked President Donald Trump’s executive order outlawing birthright citizenship —the policy automatically giving citizenship to anyone born in the U.S.—and declared it “blatantly unconstitutional,” as legal experts widely believe the president does not have the power to change the policy on his own.

Trump's order directs federal agencies to stop issuing citizenship documents to U.S.-born children of undocumented mothers. It also applies to mothers in the country on temporary visas, if the father is not a U.S. citizen or permanent resident.