Sunak — who has been in power since October 2022 — has staked his political future on slashing record levels of regular and irregular migration. His government's Safety of Rwanda (Asylum and Immigration) Bill is key to that pledge.
But the legislation has reopened divisions within the ruling Conservative Party between right-wingers and moderates, meaning Sunak faces a fight to get it on the statute books.
The plan to send migrants to Rwanda is Sunak's answer to a unanimous United Kingdom Supreme Court ruling in November that said deporting asylum seekers to the East African nation is illegal under international law.
It would compel judges to treat Rwanda as a safe country and proposes giving minister in the United Kingdom powers to disregard sections of international and British human rights legislation.
The United Nations High Commissioner for Refugees, UNHCR, reiterated this week that the legislation and a recently signed treaty with Kigali designating Rwanda "safe" were "not compatible" with international refugee law.
Meanwhile the bill has triggered deep factional Tory infighting not seen since the wranglings over Brexit.
Sunak faced down party rebels last month and won a knife-edge parliamentary vote on the Rwanda legislation. He must do so again, in a vote expected on Wednesday night.
First, beginning on Tuesday, members of parliament will debate a series of amendments to the legislation. They are unlikely to be passed but will reveal the extent of opposition that Sunak faces.
More than 50 Tory members of parliament have publicly backed right-wing amendments that toughen up the bill, including ensuring that international law does not apply.
They include two Tory deputy chairmen, who are testing Sunak's mettle for party discipline amid calls for their dismissal from the roles.
If Sunak agrees to right-wingers' demands, then the bill would almost certainly be ruined by party centrists, who oppose any violations of international law and say the legislation already pushes the limits.
Sunak said on Monday he was "talking to all my colleagues" and was "determined to get this new legislation on to the statute book so we can get our Rwanda scheme up and running".
"I'm confident that the bill we have got is the toughest that anyone has ever seen and it will resolve this issue once and for all," he told reporters.
Sunak says the law is essential to deter migrants from considering travelling to the United Kingdom via unauthorized routes, particularly on small boat crossings from France.
Around 30,000 asylum seekers crossed the English Channel on rudimentary vessels last year. Five died trying to make the journey this past weekend.
Sunak has yet to announce the date of the United Kingdom's general election but has said it will be held this year.
Some opinion polls put the main opposition Labour Party more than 20 points ahead of the Tories, suggesting the ruling party is heading for a landslide defeat.
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