The UK Supreme Court rejected Wednesday a controversial government plan to send migrants to Rwanda and upheld a lower court ruling that it was unlawful, in a major setback for Prime Minister Rishi Sunak.

A five-judge panel at the UK's highest court unanimously sided with Court of Appeal judges that the policy was incompatible with Britain's obligations under international treaties.

"We conclude that the Court of Appeal... was entitled to find that there are substantial grounds for believing that the removal of the claimants to Rwanda would expose them to a real risk of ill-treatment," they concluded. 

In a 56-page ruling, the judges agreed with the lower court's June verdict that Rwanda risked forcibly returning asylum seekers and refugees to a country where they could face persecution.

Sunak's ruling Conservatives have insisted the scheme is crucial to reduce "illegal" immigration across the Channel on small boats -- an issue set to feature prominently in the next general election.

But the finding scuppers the deal signed with Rwanda in April last year to send undocumented migrants to interim centres there, and seemingly leaves the UK leader's immigration agenda in tatters. 

It is also set to widen rifts in the Tory party between right-wing lawmakers who favour bolder action and moderates.

While conceding it was "not the outcome we wanted", Sunak said the government "has already been working in advance on a new treaty with Rwanda, which we will finalise in light of today's judgement".

"If it becomes clear that our domestic legal frameworks or international conventions are still frustrating plans at that point, I am prepared to change our laws and revisit those international relationships," he told parliament.

- 'Stop the boats' -

The Migration and Economic Development Partnership envisages sending to Rwanda anyone who has made what the government calls "dangerous or illegal journeys, such as by small boat or hidden lorries" to the UK.

The first deportees were on a plane ready to fly there in June 2022 when a last-minute European Court of Human Rights (ECHR) injunction prevented any deportations, prompting the legal challenges.

Opponents criticise the Rwanda plan as cruel, costly and difficult to implement. 

The Rwandan government said it "take(s) issue with the ruling that Rwanda is not a safe third country".

London insists the policy is essential to deter migrants crossing the Channel from France in rudimentary small vessels. 

More than 27,000 have made the journey this year -- down on the nearly 46,000 who crossed in 2022, but still far short of meeting Sunak's vow to "stop the boats".

His administration says regular and irregular immigration must be slashed to ease pressure on government-funded services, such as health and housing asylum seekers.

Britain's asylum backlog stands at 122,585, down 12 percent from a record high in February. Meanwhile net migration -- the difference between the number of people leaving the UK and those arriving -- reached a record 606,000 last year. 

Both statistics make uncomfortable reading for Conservatives, who pledged less immigration post-Brexit.

Sunak's government passed legislation in July barring any "illegal" arrivals from claiming asylum, but it relies on finding third countries to send them to. 

There is growing speculation it will now try to strike deals with other countries. 

"There is an appetite for this concept," newly-appointed interior minister James Cleverly said after the Supreme Court ruling, claiming other European countries were "following our lead".

- 'Unworkable' -

The decision is expected to renew demands from right-wingers like ex-interior minister Suella Braverman that Britain withdraw from the ECHR -- a drastic idea that Sunak has so far refused to back. 

She launched a scathing attack on Sunak the day after he sacked her, accusing him of "betrayal" over immigration and saying he had "no appetite for doing what is necessary".

Outspoken Tory party deputy chairman Lee Anderson said ministers should "ignore the laws" and deport migrants the day they arrive.

He branded the court judgment a "dark day for the British people" and said the government should "just put the planes in the air now and send them to Rwanda".

The main Labour opposition, riding high in the polls, accused Sunak of failing to "have any serious plan to tackle dangerous boat crossings".

"Labour argued from the start this plan is unworkable and extortionately expensive," said  senior MP Yvette Cooper, adding the government "failed to ensure they had a robust and workable policy".

Migrant advocates welcomed Wednesday's court ruling.

The Refugee Council said it was a "victory for the rights of men, women and children who simply want to be safe".

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