One of President Trump’s key assertions of presidential power over the southern border was ruled unlawful by a federal appeals court on Friday.

In a 2-to-1 decision, the U.S. Court of Appeals for the District of Columbia upheld an earlier ruling by a district court judge that Mr. Trump had to adhere to requirements outlined in the Immigration and Nationality Act and could not categorically deny asylum claims from people crossing from Mexico into the United States.

Existing immigration law “does not allow the president to remove plaintiffs under summary removal procedures of his own making,” Judge J. Michelle Childs wrote for the majority. She rejected the administration’s view of the law, which would allow it to “unilaterally and heedlessly return individuals even to countries where they will most certainly face persecution.”

While the ruling does not take effect immediately, it brings the administration one step closer to a requirement that it process new applications from asylum seekers. The administration now has 45 days to ask for the case to be reheard by the full appellate court. It could also appeal directly to the Supreme Court, which is already considering a separate case over whether asylum seekers can be turned back at the border before they have a chance to file an asylum claim.

Mr. Trump’s attempt to enact a sweeping ban on asylum is rooted in a proclamation he signed the day he took office for his second term. He declared that there was an “invasion” along the southern border and invoked what he called “the inherent right and duty of the executive branch to defend our national sovereignty.”

The proclamation was followed by guidance from the Department of Homeland Security that blocked people from applying for asylum other than at a designated port of entry. Those who did show up at a port of entry faced new, stringent document requirements, including medical and criminal histories. Asylum seekers, according to the proclamation, raised “health, safety, and security concerns” of sufficient gravity to require a presidential intervention.