(Bloomberg) -- Right-wing members of Rishi Sunak’s UK Conservative Party caucus said his attempt to implement a plan to deport asylum seekers to Rwanda “doesn’t go far enough,” underscoring the risk to the prime minister of a Parliamentary rebellion.

A government bill to advance the plan “provides a partial and incomplete solution to the problem of legal challenges in the UK courts” stymieing deportations to Rwanda, the “star chamber” of lawyers advising pro-Brexit Tories in the European Research Group said in their written advice. “Resolving, comprehensively, the issues raised by this analysis would require very significant amendments.”

The advice refers to legislation advanced last week by Sunak designating Rwanda a “safe” country for deportees and cutting off their routes to appeal a deportation decision. The lawyers’ verdict raises the chances that Tory right wingers will rebel on the plan either in a first vote Tuesday in the House of Commons on the general principles of the bill, or at a later stage as amendments are put forward.

For Sunak, it’s a moment of danger as he seeks to unite bitterly divided factions within his ruling Conservative Party, who are weighing up whether to derail a bill on which he has staked his reputation. In a boost to the prime minister, the One Nation group of Tory moderates late Monday recommended that its members — there are more than 100 — should back the bill. At the same time they warned they won’t swallow any moves to toughen it further.

“The most important thing at this stage is to support the bill despite our real concerns,” One Nation Chairman Damian Green said in a statement. “We strongly urge the government to stand firm against any attempt to amend the bill in a way that would make it unacceptable to those who believe that support for the rule of law is a basic Conservative principle.”

Members of Green’s bloc have expressed concerns that the legislation is too extreme, with worries centering on the potential for Britain to retreat from its international and national commitments on human rights.

Sunak last week told backbench Tories they needed to unite over the £290 million ($365 million) plan or face defeat at an election he must hold by the end of January 2025.

Some 29 rebels are needed to inflict a defeat, and ERG Chairman Mark Francois declined to say how he plans to vote. “There is a bit of time to pass before the bells ring” signaling a vote is taking place, he told reporters.

Tory MP Simon Clarke, representing the Conservative Growth Group, told reporters the ERG’s legal advice is “very concerning.” There are “clear and specific challenges to the government on whether this legislation works,” Clarke said, adding he wants Sunak to toughen up the bill.

In an effort to persuade doubters, Sunak on Monday published a summary of the legal advice underpinning his plan — an unusual move for the government, which normally keeps such documents confidential. 

“The government’s approach is tough but fair and lawful,” according to the document. “It has a justification in the UK’s constitution and domestic law, and it seeks to uphold our international obligations.”

Sunak argues the legislation would lift most of the legal barriers that have blocked deportation flights from taking off since the plan was first advance last year by then Prime Minister Boris Johnson. The government advice said the legislation leaves asylum seekers some “very limited scope” to appeal against deportation if they could demonstrate they faced a “real risk of serious and irreversible harm.” There wouldn’t be a “clear legal basis” for the government to proceed without that, according to the advice.

While the current premier needs to get the legislation through the Commons to show he still controls a governing majority, he has stopped short of making its passage a test of confidence in his leadership. But some of his MPs say it is a vote of confidence in all but name. 

The deportation deal with Rwanda is the centerpiece of Sunak’s effort to curb a surge in migrants crossing the English Channel in small boats and claiming asylum: more than 29,000 have done so this year, and more than 45,000 made the crossing in 2022. Ministers argue deportation to Rwanda will act as a deterrent to migrants undertaking the crossing in the first place. 

But in the latest of a series of legal setbacks, the UK Supreme Court deemed the plan unlawful, prompting Sunak to devise the latest legislation allowing some human rights law to be sidestepped, but stopping short of removing the UK from the reach of the European Convention on Human Rights, as some right-wing Tories would like.

Government officials acknowledged Sunday that the legislation might not reach a final vote before a planned Christmas recess on Dec. 19 and debate could spill into the new year. 

--With assistance from Alex Wickham and Ellen Milligan.

(Updates with position of Tory moderates starting in fourth paragraph.)

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