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.
