British Prime Minister Rishi Sunak faces the biggest parliamentary test of his premiership on Tuesday when lawmakers vote on his divisive plan to send asylum seekers to live in Rwanda.

The UK Supreme Court ruled last month that Rwanda was an unsafe place to send those arriving in small boats on England's southern coast, and that the government's plan would breach British and international law.

In response, Sunak has agreed a new treaty with the east African country and brought forward emergency legislation designed to override domestic and international human rights law that would prevent deportations.

The legislation has deeply divided his party, alienating both moderates, who are worried about Britain breaching its human rights obligations, and right-wing politicians, who contend it does not go far enough.

Some right-wing Conservatives called for a full rewrite of the legislation on Monday, saying it would not stop asylum seekers from successfully appealing against deportation.

One member of parliament in this group said they had not decided whether to abstain or vote against the government.

Parliament will hold the first vote on the draft law on Tuesday evening. It would only take about 30 Conservative members of parliament to vote with opposition parties to defeat the bill.

Even if it passes, Sunak is likely to face attempts to toughen it up with amendments at later stages, as well as opposition in the House of Lords, the unelected upper chamber.

Defeat would be a huge embarrassment for Sunak - no government has lost a vote at this early stage in the parliamentary process since 1986 - and it would severely weaken his authority over his party.

The battle over the legislation has echoes of parliamentary showdowns over Brexit from 2017-19, when former Prime Minister Theresa May suffered repeated defeats following rebellions by large numbers of Conservative politicians, eventually leading to her exit.

Sunak, who replaced Liz Truss as prime minister last year after she was forced out, has made stopping boat-arrivals one of his biggest priorities. The plan is being closely watched by other countries considering ways to halt immigration.

This year about 29,000 asylum seekers have arrived on the southern coast of England, down around one-third compared with last year.

But they remain a highly visible symbol of the government's failure to control Britain's borders - a key promise of campaigners for Brexit.

The government has already paid 240 million pounds ($301 million) to Rwanda even though it only has the capacity to settle hundreds of refugees a year from Britain and no one has yet been sent to the country.

In a day of political drama in Westminster on Monday, lawmakers from different factions of the governing Conservative Party held meetings throughout the day to decide how to vote.

In a boost late on Monday, a centrist group of lawmakers in the One Nation faction recommended that its members back the bill but warned that they would not accept any amendments that toughened the legislation at a later stage.