The US Supreme Court has allowed the government to severely limit the ability of migrants to claim asylum.
The policy bars people arriving at the US southern border from seeking protection if they failed to do so in a country they passed through en route.
Legal challenges continue but the ruling means for now it can be enforced nationwide.
The plan will affect tens of thousands of Central American migrants who travel north, often on foot, through Mexico.
The Trump administration unveiled the new asylum policy in July but it was almost immediately blocked from taking effect by a lower court ruling by a judge in San Francisco.
Curbing migration levels has been a key goal of Donald Trump’s presidency and forms a major part of his bid for re-election in 2020.
He hailed the Supreme Court’s decision as a major victory.
