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 policy would breach British and international law.

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

The move 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.

"We believe the best solution here is we should pause the legislation today, we should come back with a new bill," one Conservative lawmaker, Simon Clarke, told BBC Radio.

Lawmakers on the right of the party, who have not said whether they will abstain or vote against the government, believe the bill will still allow asylum seekers to appeal against deportation.

The government's legal advice said access to courts formed part of Britain's tradition of liberty and justice, available even in wartime.

The British parliament will hold the first vote on the 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.