Some asylum seekers who cross the Channel to the UK will be given a one-way ticket to Rwanda under new government plans.
The pilot scheme will focus on single men arriving on boats or lorries.
Prime Minister Boris Johnson said the £120m scheme would "save countless lives" from human trafficking.
Refugee organisations have criticised the plan as cruel. They questioned its cost and impact and raised concerns about Rwanda's human rights record.
Mr Johnson said action was needed to stop "vile people smugglers" turning the ocean into a "watery graveyard", with the plan designed to break their business model.
"Our compassion may be infinite but our capacity to help people is not," he said. "We can't ask the British taxpayer to write a blank cheque to cover the costs of anyone who might want to come and live here."
Those who succeed in making it to the UK "will be taken not to hotels at vast public expense", the prime minister said, and instead would be housed in detention centres.
Speaking in Kent, he said the new plan would "over time prove a very considerable deterrent".
Labour leader Sir Keir Starmer described the scheme as "unworkable" and "extortionate", claiming it was an attempt to distract from Mr Johnson's "partygate" fine.
Last year, 28,526 people are known to have crossed in small boats, up from 8,404 in 2020. About 600 people made the crossing on Wednesday - the figure could reach 1,000 a day in coming weeks, Mr Johnson said.
The number of people who can be relocated will be "unlimited", said Mr Johnson.
Rwanda will have the "capacity to resettle tens of thousands of people in the years ahead", including those who have arrived "illegally" since the start of the year, he said.
"We cannot sustain a parallel illegal system," said the prime minister. "Our compassion may be infinite, but our capacity to help people is not."
Home Secretary Priti Patel, who travelled to Rwandan capital Kigali to sign the deal, said it was a "global first and it will change the way we collectively tackle illegal migration".
She said the "vast majority" of those arriving in the UK "illegally" would be considered for relocation to Rwanda.
Recent research by Ipsos Mori suggested 60% of the public were dissatisfied with the government's migration policy - more than half of which cited Channel crossings.
BBC home editor Mark Easton, reporting from Rwanda, said ministers would face legal hurdles and substantial costs to launch the scheme.
Precise details of the plan are yet to be confirmed - but he said the trial would be restricted to mostly single men.
Under the proposal, Rwanda would take responsibility for the people who made the more than 4,000-mile journey, put them through an asylum process, and at the end of that process, if they were successful, they would have long-term accommodation in Rwanda.
The BBC has seen accommodation the asylum seekers would be housed in, thought to have enough space for about 100 people at a time and to process up to 500 a year.
The Rwandan government said migrants would be "entitled to full protection under Rwandan law, equal access to employment, and enrolment in healthcare and social care services".
The UK Home Office believes existing asylum law will be enough to implement the plan, but questions remain about the legality of the scheme.
Mr Johnson said the plan was "fully compliant" with international law, but acknowledged he expected it to be subject to challenge in the courts and from a "formidable army of politically-motivated lawyers".
'Something's got to give'
BBC Radio 5 Live listeners shared their views with Nicky Campbell:
- "What's crueller - that or being drowned? If it saves one life, surely it's got to be better than what's happening with these migrants now" - Lorraine, Dartington
- "How are we treating these humans? Are we suddenly saying those coming from Ukraine, their lives are better value than those coming from certain other countries? I think it's abhorrent" - Hamid, Bicester
- "I feel it's a real shame. We seem to have eroded this idea of fairness in our nation. We seem to no longer want to be the nation that wants to help other people. It seems a shame we've lost those values in our country" - Chris, Devon
- "Let's forget about the optics and actually look at the reality. People smugglers will keep trying to find different ways until we break it. That's the cruel part here... I applaud the government for having some backbone" - Martin, Bedford
British Red Cross executive director Zoe Abrams said the humanitarian network was "profoundly concerned" about the scheme and that "the financial and human cost will be considerable".
Enver Solomon, chief executive of the Refugee Council, said the charity was "appalled by the government's cruel and nasty decision", which it said would "do little" to deter people from coming to the UK.
Miranda Butler, a barrister who specialises in immigration law, said there were "serious questions" about the risks faced by migrants under this "expedited process".
The Lib Dems said the government was "slamming the door" in the face of refugees, while the SNP's Ian Blackford described the plan as "absolutely chilling".
Questions have been raised over the human rights record of the Rwandan government and its president, Paul Kagame.
Last year, the UK government expressed concern at the United Nations over "continued restrictions to civil and political rights and media freedom" in Rwanda.
But Mr Johnson described Rwanda as being one of the safest countries in the world.
Other countries - including Ascension Island and Gibraltar - had also been considered.
The prime minister also announced:
- Asylum seekers who are resettled in the UK will be spread more evenly across local authorities
- Plans to hand operational control of the Channel to the Royal Navy
- £50m in funding for new equipment and specialist personnel for Channel operations
- A new government facility to house migrants, described as a reception centre, in Linton-on-Ouse, North Yorkshire
- A maximum sentence of life imprisonment for people smugglers
The government's Nationality and Borders Bill is making its way through Parliament, but time is running out to pass it into law after a series of defeats in the House of Lords.
MPs are currently on a break, but when they return, they are due to review a series of amendments, including one about powers to offshore asylum claims.
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