Here are some of the problems Sunak will have to overcome before his promise to send those who arrive in Britain without permission to the East African nation becomes a reality.

PARLIAMENT

After hours of debate, lawmakers in parliament's lower House of Commons passed the government's 'Safety of Rwanda Bill' by 320 votes to 276, with 11 right-wing Conservatives rebelling.

The essence of the bill is to override a decision by the UK Supreme Court which in November declared the policy of sending asylum seekers to Rwanda unlawful.

It now passes to the unelected upper chamber, the House of Lords, where Sunak does not command an automatic majority and where many peers could seek to oppose a bill which critics say might lead to Britain breaching international law.

The Lords could try to make amendments to the bill meaning it passes back and forth between the two parliamentary houses in what is known as a "ping pong" process.

Normally, peers avoid frustrating the will of the elected chamber and, under the "Salisbury doctrine", do not vote down a bill mentioned in a governing party's election manifesto. However, the Rwanda scheme was not included in Conservatives' 2019 manifesto, meaning peers could in theory refuse to pass the bill without amendments.

In such extreme circumstances, the law would be delayed for a year, which would mean it could not be passed until after the next election.

In a sign of the likely opposition, a Lords committee that scrutinises international agreements said a treaty agreed between the British government and Rwanda last month that seeks to prevent the East African sending asylum seekers anywhere other than back to Britain should not be ratified.

The committee said the treaty lacks safeguards including a system to prevent people being sent back to their country of origin where they could be at risk of ill-treatment.

LEGAL CHALLENGES

Sunak has described the bill as Britain's toughest proposed immigration law. But his right-wing critics say it does not go far enough and that there are loopholes which will be seized upon by asylum seekers to stop their deportation.

The first planned flight to take migrants to Rwanda was blocked when the European Court of Human Rights (ECHR) issued "interim measures" under its Rule 39 provision - effectively a temporary emergency injunction.

Sunak says the bill will make it "vanishingly rare" for any asylum seeker to be able to block deportation by an appeal to the courts, and would also mean his immigration minister had the power to decide whether to comply with any ECHR orders.

But some legal commentators have cast doubt on both those assertions. They argue the bill itself could face challenges in the UK courts, that it would allow individual appeals, and that Britain would still have an obligation under international law to abide by any order from the ECHR.

OTHER ISSUES

According to information issued by the British government last week, the Rwandan government will need to pass a new asylum law "in the coming months" in order to ratify a new treaty agreed with the United Kingdom to allow the scheme to go ahead.

Last month, British media also reported that the government was struggling to find airlines which would agree to provide flights to take the asylum seekers to Rwanda.

While Sunak has promised the deportations would begin in spring this year, it is not clear whether, if the bill becomes law, how quickly deportation flights would begin.

(Reporting by Michael Holden, Editing by Elizabeth Piper and Alistair Bell)