Sunak said the government would seal a legally binding treaty with Rwanda that would address the court’s concerns, and would then pass a law declaring Rwanda a safe country.

Sunak suggested that if legal challenges to the plan continued, he was prepared to consider leaving international human rights treaties — a move that would draw strong opposition and international criticism.

Britain and Rwanda signed a deal in April 2022 to send migrants who arrive in the U.K. as stowaways or in boats to the East African country, where their asylum claims would be processed and, if successful, they would stay.

Britain’s government argues that the policy will deter people from risking their lives crossing one of the world’s busiest shipping lanes, and would break the business model of people-smuggling gangs. No one has yet been sent to the country as the plan was challenged in the courts.

Opposition politicians, refugee groups and human rights organizations say the plan is unethical and unworkable. Charity ActionAid U.K. called the Supreme Court ruling a vindication of “British values of compassion and dignity.” Amnesty International said the government should “draw a line under a disgraceful chapter in the U.K.’s political history.”

Announcing the unanimous decision, President of the Supreme Court Robert Reed said Rwanda had a history of misunderstanding its obligations toward refugees and of “refoulement” — sending claimants back to the country they had sought protection from.

The judges concluded “there is a real risk that asylum claims will not be determined properly, and that asylum-seekers will in consequence be at risk of being returned directly or indirectly to their country of origin.”

“In that event, genuine refugees will face a real risk of ill-treatment,” they said.

The U.K. government has argued that while Rwanda was the site of a genocide that killed more than 800,000 people in 1994, the country has since built a reputation for stability and economic progress.

Critics say that stability comes at the cost of political repression. The court’s judgment noted human rights breaches including political killings that had led U.K. police “to warn Rwandan nationals living in Britain of credible plans to kill them on the part of that state.” They said Rwanda has a 100% rejection record for asylum-seekers from war-torn countries including Syria, Yemen and Afghanistan.

The Rwandan government insisted the country is a safe place for refugees.

“Given Rwanda’s welcoming policy and our record of caring for refugees, the political judgments made today were unjustified,” it said in a statement.

Rwandan opposition leader Frank Habineza, however, said Britain shouldn’t try to offshore its migration obligations to the small African country.

“The U.K. should keep the migrants or send them to another European country, not to a poor country like Rwanda. I really think it’s not right (for) a country like the U.K. to run away from their obligations,” Habineza told the AP in Kigali.

Much of Europe and the U.S. is struggling with how best to cope with migrants seeking refuge from war, violence, oppression and a warming planet that has brought devastating drought and floods.

Though Britain receives fewer asylum applications than countries such as Italy, France or Germany, thousands of migrants from around the world travel to northern France each year in hopes of crossing the English Channel.

More than 27,300 have done that this year, a decline on the 46,000 who made the journey in all of 2022. The government says that shows its tough approach is working, though others cite factors including the weather.

The Rwanda plan has cost the British government at least 140 million pounds ($175 million) in payments to Rwanda before a single plane has taken off. The first deportation flight was stopped at the last minute in June 2022, when the European Court of Human Rights intervened.

The case went to the High Court and the Court of Appeal, which ruled that the plan was unlawful because Rwanda is not a “safe third country.” The government unsuccessfully challenged that decision at the Supreme Court.

Sunak took comfort from the court’s ruling that “the structural changes and capacity-building needed” to make Rwanda safe “may be delivered in the future.” The U.K. government says its legally binding treaty will compel Rwanda not to send any migrants deported from the U.K back to their home countries.

The prime minister is under pressure from the right wing of the governing Conservative Party to take even more dramatic action to “stop the boats.” Former Home Secretary Suella Braverman, who was fired by Sunak on Monday, has said the U.K. should leave the European Convention on Human Rights if the Rwanda plan was blocked.

Sunak said at a news conference he was prepared to “revisit those international relationships to remove the obstacles in our way.”

“I will not allow a foreign court to block these flights,” he said.

Legal experts said leaving or ignoring international treaties would be an extreme move. Joelle Grogan, a senior researcher at the U.K. in a Changing Europe think tank, said leaving the European Convention would make Britain “an outlier in terms of its standards and its reputation for human rights protection.”

“The only reason in which you would leave the ECHR is if you wanted to start sending asylum-seekers to unsafe countries where they face threats to their life,” she said.