In a blow to Prime Minister Rishi Sunak's government and his pledge to "stop the boats," London's Court of Appeal concluded in June the scheme to send tens of thousands of migrants more than 4,000 miles (6,400 km) to East Africa was not lawful, saying Rwanda could not be treated as a safe third country.
On Monday, government lawyers will argue at the Supreme Court this ruling was wrong, while those representing migrants from Syria, Iraq, Iran, Vietnam and Sudan want the judges to conclude the scheme itself is flawed.
The stakes for Sunak are high, as he has made dealing with immigration one of his five priorities. Successfully addressing the issue could revive his Conservative Party's ailing fortunes as it languishes some 20 points behind in opinion polls ahead of an election expected next year.
"A government that doesn't deliver on what you promised will always get punished. We need to get a grip on this issue," Conservative lawmaker Brendan Clarke-Smith told Reuters at the party's annual conference this week.
Sunak and his ministers argue that the Rwanda scheme, launched last year by then Prime Minister Boris Johnson, would smash the business model of human traffickers, and deter people from the perilous cross-Channel journey in inflatable boats and dinghies. Six people drowned in August while 27 perished in November 2021.
Opponents say it is immoral, expensive and simply will not work. Their number includes human rights groups, lawmakers, including some Conservatives, and the Archbishop of Canterbury, the head of the Anglican communion. Even King Charles is reported by media to have privately expressed reservations.
The fate of the scheme now lies in the hands of five judges, including the Supreme Court's President Robert Reed, who will begin hearing mainly technical legal argument over three days starting on Monday.
Like many nations across Europe, Britain has been wrestling with how to deal with the influx of migrants often fleeing war zones in the Middle East, Africa and Afghanistan.
In a speech on Tuesday to Conservative Party members, interior minister Suella Braverman said that a "hurricane" of migrants was threatening to hit Britain and she vowed to stop what she called "bogus asylum seekers."
Immigration was one of the main factors behind the 2016 vote to leave the European Union, with the promise that Britain would take back control of its borders.
Overall net migration has continued to rise, reaching a record high of 606,00 last year. This year, more than 25,000 people have arrived in Britain on small boats, while a record 45,755 were detected in 2022.
Britain says the cost of its broken asylum system, with some 135,000 people waiting for a decision, is more than 3 billion pounds ($3.6 billion) a year. Housing some of those migrants in hotels costs about 6 million pounds a day.
A new law, passed in July, now makes it a legal duty on the interior minister to deport migrants arriving without permission either back to their homeland or to a safe third country. Britain has only signed such an agreement with Rwanda.
Sending each asylum seeker to the African country would cost on average 169,000 pounds, the government has said.
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