The U.S. Supreme Court on Thursday gave 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 U.S. hearings on their asylum claims.

The justices, 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 program.

The ruling bolsters Biden as he pursues what he calls a more "humane" approach at the southern border even as Republicans blame him for what they portray as an immigration crisis.

The justices concluded that the New Orleans-based 5th U.S. 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 U.S. 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 U.S.-Mexican border, changing longstanding U.S. 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 returning them to Mexico.

Biden's fellow Democrats and immigration advocates have criticized Trump's policy, saying migrants stuck in Mexican border cities have faced kidnappings and other hazards.

Roberts was joined by fellow conservative Justice Brett Kavanaugh and the three liberal justices in the ruling. In dissent, Justice Samuel Alito - joined by fellow conservative Justices Clarence Thomas and Neil Gorsuch - said Congress never meant for the government to release immigrants and simply hope they "will show up for the hearing."

The ruling also faulted the 5th Circuit for voiding the administration's June 2021 decision to end Trump's program. The 5th Circuit found that Biden's administration had failed to properly explain its rescinding of Trump's policy in violation of federal administrative law. But the Supreme Court found that the June 2021 decision was superseded by a new, more detailed one issued by the administration four months later.

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.

At issue in the case was the meaning of a provision of a 1996 U.S. immigration law that stated that U.S. 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.

Kavanaugh, in a concurring opinion, said that every president since the late 1990s has allowed immigrants into the United States to await their proceedings.

Texas Governor Greg Abbott, a Republican, criticized the ruling, saying it "will only embolden the Biden administration's open border policies."

Immigrant rights groups called the ruling a victory.

"The U.S. for generations has been a refuge for those fleeing danger and persecution," said Marielena Hincapi?, 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 U.S.-Mexico border has reached record highs recently. Republicans contend that the "remain in Mexico" policy effectively deterred unlawful migration.

After a judge ruled in favor of Texas and Missouri, reinstating the program, the Supreme Court last August refused the Biden administration's request to block that decision while it appealed. The 5th Circuit ruled in December that because the government lacks the capacity to detain all migrants eligible for admission pending a hearing, it must maintain "remain in Mexico."

Thursday's decision came on the final day of rulings for the court's current nine-month term.