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The U.K.'s Supreme Court issued a landmark ruling today that found the British government's plan to offshore its asylum policy was illegal. The proposal had been to send those seeking asylum in Britain to Rwanda, but judges determined that authorities in the East African nation might not sufficiently ensure the safety of the deportees, as Willem Marx reports.

WILLEM MARX, BYLINE: After 18 months of legal challenges, the conservative government's controversial idea to send thousands of asylum-seekers to a third country faced its final test. The policy was first introduced at a time when tens of thousands of migrants began arriving on British shores via dangerous small boat journeys from northern France and was designed to deter others in the future who might be considering their own irregular route to Britain.

At the center of today's ruling was a legal term - refoulement. It refers to the possibility that asylum-seekers sent to Rwanda could one day be returned to their country of origin and, once there, face possible persecution. Delivering the decision was the court's president, Robert Reed, who explained the judges' thought process.

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ROBERT REED: Changes needed to eliminate the risk of refoulement may be delivered in the future, but they have not been shown to be in place now.

MARX: The high number of migrant arrivals had threatened to overwhelm the U.K.'s poorly funded and understaffed immigration system - so often at the heart of heated political debate. The plan won some public support but attracted international condemnation from human rights groups and even the United Nations Refugee Agency. And on a practical level, authorities were unable to even implement the policy thanks to legal challenges from those facing deportation. The ruling was far from abstract for some of those involved today. Sophie Lucas from law firm Duncan Lewis has several clients that stand to benefit immediately, she says, from this decision.

SOPHIE LUCAS: Many of our clients are survivors of torture or trafficking. The last year or so, they have been left in limbo. They are incredibly relieved today that they can start dreaming of a future which does not involve being removed to Rwanda.

MARX: Within hours of the ruling, Prime Minister Rishi Sunak acknowledged to a rowdy parliament that the court had blocked his current preferred course of action.

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PRIME MINISTER RISHI SUNAK: There are further elements that they want additional certainty on and noted changes...

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SUNAK: ...And noted that changes can be delivered in the future to address those issues.

MARX: But he insisted he already had several alternatives and would seek to circumvent this latest setback to his polarizing policy.

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SUNAK: The government has been working already on a new treaty with Rwanda, and we will finalize that in light of today's judgment.

MARX: His main political opponent, Labour Party leader and former human rights lawyer Keir Starmer, delivered his own verdict, and it was scathing.

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KEIR STARMER: He was told over and over again that this would happen - that it wouldn't work and it was just the latest Tory gimmick.

UNIDENTIFIED PERSON: Gimmick...

STARMER: But he bet everything on it, and now he's totally exposed. The central pillar of his government has crumbled beneath him.

UNIDENTIFIED PERSON: Yeah.

STARMER: Does he want to apologize to the country for wasting 140 million pounds of taxpayer cash and wasting his entire time in office?

MARX: The Rwanda policy, as it became known, was only ever meant to be a short-term plan - a five-year trial that would see Rwanda receive $150 million from British taxpayers. But like the first aircraft chartered last summer to transport asylum-seekers to East Africa, it has never really taken off.

For NPR News, I'm Willem Marx from London.

(SOUNDBITE OF BADBADNOTGOOD AND GHOSTFACE KILLAH SONG, "STREET KNOWLEDGE") Transcript provided by NPR, Copyright NPR.

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