The US Supreme Court has given a major boost to President Joe Biden's drive to end a hardline immigration policy begun under his predecessor Donald Trump that forced tens of thousands of migrants to stay in Mexico to await US hearings on their asylum claims.
The justices on Thursday, in a 5-4 ruling authored by Chief Justice John Roberts, overturned a lower court's decision requiring Biden to restart Trump's "remain in Mexico" policy after the Republican-led states of Texas and Missouri sued to maintain the programme.
The justices concluded the New Orleans-based 5th US Circuit Court of Appeals erred in finding that federal immigration law required sending migrants back to Mexico so long as there was not enough space to detain them in the United States.
"The problem is that the statute does not say anything like that," Roberts wrote, adding that the 5th Circuit's decision also mistakenly imposed a "significant burden" upon the US government's ability to conduct diplomatic relations with Mexico.
Trump's administration adopted the policy, formally called the "Migrant Protection Protocols," in 2018 in response to an increase in migration along the US-Mexican border, changing longstanding US practice.
It prevented certain non-Mexican migrants, including asylum seekers fearing persecution in their home countries, from being released into the United States to await immigration proceedings, instead of returning them to Mexico.
Biden's fellow Democrats and immigration advocates have criticised Trump's policy, saying migrants stuck in Mexican border cities have faced kidnappings and other hazards.
Biden suspended the "remain in Mexico" policy in January 2021 shortly after taking office and acted to rescind it five months later.
Roughly 68,000 people fell under the policy from the time it took effect in 2019 until Biden suspended it.
Rights groups hail ruling
At issue in the case was the meaning of a provision of a 1996 US immigration law that stated that US officials "may return" certain immigrants to Mexican territory pending immigration proceedings.
Texas and Missouri have said this provision must be used because the United States lacks detention space for migrants. Biden's administration said the provision was clearly discretionary.
For migrants not posing a security risk, immigration law separately allows their release into the United States for humanitarian reasons or "significant public benefit" pending a hearing, a practice officials have followed for decades.
Texas Governor Greg Abbott, a Republican, criticised the ruling, saying it "will only embolden the Biden administration's open border policies."
Immigrant rights groups called the ruling a victory.
"The US for generations has been a refuge for those fleeing danger and persecution," said Marielena Hincapie, executive director of the National Immigration Law Center, urging Biden's administration to "move swiftly to permanently end every facet of the human rights disaster that is 'remain in Mexico.'"
The number of migrants caught crossing the US-Mexico border has reached record highs recently. Republicans contend that the "remain in Mexico" policy effectively deterred unlawful migration.