The UK government wants to send some asylum seekers to Rwanda, but the policy has been ruled unlawful by the Supreme Court.
The Court said there was a risk that asylum seekers sent there could be returned to their home country, which would break UK and international human rights laws.
But the prime minister insists the policy will go ahead.
What was the Rwanda asylum plan?
The five-year trial - announced in April 2022 - would have seen some asylum seekers sent to Rwanda to claim asylum there.
Under the plan, they could be granted refugee status to stay in Rwanda. If not, they could apply to settle there on other grounds, or seek asylum in another "safe third country".
The government said "anyone entering the UK illegally" after 1 January 2022 could be sent there, with no limit on numbers.
Rwanda could also have asked the UK to take in some of its most vulnerable refugees.
The UK has already paid the Rwandan government £140m, but no asylum seeker has actually been sent to the country. The first flight was scheduled to go in June 2022, but was cancelled after legal challenges.
Why did the government want to send asylum seekers to Rwanda?
The government said the policy would deter people arriving in the UK through "illegal, dangerous or unnecessary methods", such as on small boats across the English Channel.
More than 45,700 people used this route to come to the UK in 2022, the highest figure since records began.
In January the PM said "stopping the boats" was one of his key priorities.
As of 13 November, the number of small boat crossings in 2023 was a third lower than at the same point the year before. But it is not clear which government policies have contributed to that fall.
What did the Supreme Court say about the Rwanda policy?
The UK Supreme Court ruled unanimously that the Rwanda scheme was unlawful.
Five top justices said the Court of Appeal had been right to conclude in June that there had not been a proper assessment of whether Rwanda was a safe country for asylum seekers.
Lord Reed, the Court president, said there was strong evidence to believe that genuine refugees sent to the country could be at risk of being returned to their home countries where they could face persecution. In law, this is called "refoulement".
It breaches part of the European Convention on Human Rights (ECHR), which prohibits torture and inhuman treatment. The UK is a signatory to the ECHR.
The judges said the policy also breached safeguards in three British laws passed by Parliament over the last 30 years.
And they cited Rwanda's poor human rights record, and its past treatment of refugees.
The Rwandan government rejected the judges' conclusions, saying: "We take our humanitarian responsibilities seriously, and will continue to live up to them."
What has the government said?
Prime Minister Rishi Sunak said he remained "completely committed to stopping the boats", and was determined to "end the merry-go-round" of legal challenges.
He told MPs that the government was negotiating a new treaty with Rwanda that would protect against refoulement, and was prepared to change the law to ensure the policy went ahead.
Immigration Minister Robert Jenrick later said it was "absolutely critical that flights go off to Rwanda in the spring".
Emergency legislation could declare that Rwanda is a safe country, but legal experts have questioned how that might work.
Former Supreme Court judge Lord Jonathan Sumption told the BBC the prime minister's plan was "profoundly discreditable", and would still be a breach of the government's international law obligations.
"It would be constitutionally a completely extraordinary thing to do, to effectively overrule a decision on the facts, on the evidence, by the highest court in the land," he said.
Where is Rwanda?
Rwanda is a small land-locked country in east-central Africa, 4,000 miles (6,500km) south-east of the UK.
It borders Burundi, the Democratic Republic of the Congo (DRC), Tanzania and Uganda.
With a landmass about one-tenth of the size of the UK, it has a population of 13.8 million.
President Paul Kagame hopes to win a fourth term in 2024, which would extend his presidency to nearly three decades.
He won the last presidential election in 2017 with nearly 99% of the vote, but critics accuse him of supressing his political opponents.
According to Human Rights Watch, "Rwanda is a country where it's very dangerous to oppose the government".
Would the plan save the government money?
The government has not provided a total cost for the scheme.
An economic-impact assessment prepared for the government's Illegal Migration Bill estimated that removing each individual to a third country, such as Rwanda, would cost £63,000 more than keeping them in the UK.
That is the difference between the total cost of removing an individual - estimated to be £169,000 - and the £106,000 spent on housing support if they remain in the UK.
The latter figure includes a payment to the third country of around £105,000 per person, as well as £22,000 for flights.
The Home Office said no cost would be incurred if the policy prevented an individual from entering the UK illegally.
But it acknowledged it could not say how many people would be deterred.
The UK's asylum system costs £3bn a year. About £8m a day is spent on hotel accommodation for refugees and asylum seekers.
Critics say the daily cost is so high because of the time taken to decide on applications, and a ban on asylum seekers working while waiting for confirmation of their status.