A US federal court has ruled that the Trump administration must halt its policy of requiring Central American asylum-seekers to wait in Mexico pending US approval.
The ruling from the San Francisco-based appeals court temporarily upends Mr Trump's policy
The so-called "Remain in Mexico" policy is touted by supporters as key to reducing illegal migration.
Friday's decision is expected to be challenged in the US Supreme Court.
In the past year, some 60,000 migrants have been sent back to Mexico.
The border cities where migrants wait for months are suffering from growing crime rates. According to charity Human Rights First, there have been more than 800 reports of kidnapping, rape and other violent crimes against returned migrants.
The policy, officially called the Migrant Protection Protocols (MPP), took effect in January 2019.
It requires migrants entering through the southern border to wait in Mexico as US immigration courts hear their cases.
Asylum-seekers were previously permitted to remain in the US pending the outcome of their case, which sometimes takes years to resolve. Mr Trump has claimed that allowing asylum-seekers to remain in the US made it more likely that they would skip the legal process.
The policy does not apply to Mexican citizens.
What did the court say?
The Ninth Circuit Court of Appeals ruled on Friday that the MPP is "invalid in its entirety" due to its inconsistencies with existing laws.
The three-judge panel ruled that a block on the MPP, which was granted by a lower court, was "not an abuse of discretion".
In a separate decision, the court agreed to stop another major Trump administration policy denying asylum to anyone caught entering the US illegally from Mexico.
The numbers of migrants entering the US illegally has declined sharply in recent months.
In an October report, the Department of Homeland Security (DHS) called the Remain in Mexico policy "an indispensible tool in addressing the ongoing crisis at the southern border and restoring integrity to the immigration system".
It was unclear whether the decision meant migrants being held in Mexico would immediately be allowed to cross into the US. The DHS has not returned a BBC request for comment.