The UK’s controversial policy to deport asylum seekers to Rwanda is unlawful, the nation’s highest court ruled, dashing Prime Minister Rishi Sunak’s central plan to stem cross-channel arrivals in small boats.

The Supreme Court decision on Wednesday is a major embarrassment for the Conservative government and extinguishes hope that Sunak can make good on the central plank of his tough immigration reform. Sunak’s call to “stop the boats” focused on preventing asylum seekers traveling to the UK across the English Channel from France, and is one of five key pledges he’s asked voters to judge him by.

Judge Robert Reed, the president of the Supreme Court, ruled that there was a real risk that asylum seekers processed by the Rwandan state may be sent back to their home countries.

The proposal, which involves plans to fly asylum seekers arriving in Britain some 4,000 miles (6,400 kilometers) to the African nation, spurred a series of legal challenges. Refugees and charities challenging the plans said it didn’t align with human-rights conventions and that Rwanda wasn’t a safe place to send asylum seekers.

“The court of appeal was entitled to conclude that there were substantial grounds for believing that asylum seekers would be at real risk,” of ill treatment, the five judge panel ruled unanimously.

Just days after a reshuffle of his senior cabinet ministers that saw Home Secretary Suella Braverman fired and ex-premier David Cameron make a shock return to politics, the ruling is a new blow to the prime minister as he pushes to reassert his authority before a general election expected next year.

Stopping the boats carrying migrants from France was one of the five key pledges of Sunak’s premiership, along with cutting NHS waiting lists and restoring the UK economy, staking his political fate in-part on the Rwanda policy.

The verdict is also likely to energize the Tory right. The recently-fired Braverman was one of the most vocal supporters of the Rwanda plan and in an excoriating resignation letter she accused Sunak of being “weak” and “manifestly and repeatedly failed to deliver” on his commitments to her, including on migration. She also slammed him for failing to prepare a “Plan B” in the event of losing Wednesday’s case.

Despite all the rhetoric, asylum seekers are still arriving in significant numbers. The asylum caseload in the UK rose to 215,500 as of June, according to government data.

Sunak argues that even without flights taking off, his Rwanda plan is working as a deterrent. Home Office and Border Force data show that some 26,699 immigrants crossed the channel in small boats in the first ten months of the year — that’s almost exactly a third down from the almost 40,000 who had done so by the same stage last year.

--With assistance from Ellen Milligan and Jonathan Browning.

(Updates with ruling quote in the fifth paragraph)