The trial will involve mostly 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, questioned its cost and efficacy, and raised concerns about Rwanda’s human rights record.

In a speech in Kent, Mr Johnson argued action was needed to stop “vile people smugglers” turning the ocean into a “watery graveyard”, adding the “humane and compassionate” plan was designed to break their business model.

He said he wanted to make clear to people arriving on the Kent coast they were better off taking legal routes and that the new plan would “over time prove a very considerable deterrent”.

Last year, 28,526 people are known to have crossed in small boats, up from 8,404 in 2020.

Around 600 people made the crossing on Wednesday, and Mr Johnson said the figure could reach 1,000 a day within weeks.

Mr Johnson said the scheme would be uncapped, affect those who arrived illegally since 1 January, and potentially involve tens of thousands in the coming years.

The BBC has seen accommodation the asylum seekers will be housed in, thought to have enough space for around 100 people at a time and to process up to 500 a year.

“We cannot sustain a parallel illegal system,” the prime minister said. “Our compassion may be infinite, but our capacity to help people is not.”

Home Secretary Priti Patel travelled to the Rwandan capital Kigali to sign the deal.

BBC home editor Mark Easton, reporting from Rwanda, explained ministers 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 the British authorities believe are inadmissible.

Under the proposal, Rwanda would take responsibility for the people who make the more than 4,000 mile journey, put them through an asylum process, and at the end of that process, if they are successful, they will have long-term accommodation in Rwanda.

The Rwandan government said migrants will 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.

Questions have also been raised over the human rights record of the Rwandan government and its president, Paul Kagame.

Last year, the UK government itself expressed concern over “continued restrictions to civil and political rights and media freedom” in Rwanda at the United Nations.

However, Mr Johnson described Rwanda as being one of the safest countries in the world.

The prime minister also announced:

The partnership with Rwanda is the centrepiece of a wider policy blitz to deal with what has been a humiliation for ministers who promised Brexit would mean control of Britain’s borders.

Instead, record numbers of asylum seekers have been turning up in dinghies beneath the white cliffs of Dover. This year has already seen 4,578 arrivals and looks set to be a new record.

Sending asylum seekers to Rwanda, however, is likely to prove hugely controversial and legally fraught.

Critics point to Rwanda’s poor human rights record. At the UN last year, the UK demanded investigations into alleged killings, disappearances and torture.

Ministers will have to explain why Rwanda is the right place to entrust with protecting the human rights of vulnerable asylum seekers who hoped the UK would protect them.

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.

The government wants to introduce new laws – under the Nationality and Borders Bill – to make it easier for the UK to send refugees to another country for their asylum claim to be processed.

But the country must be “safe” and the UK has also signed up to two key international treaties guaranteeing refugees’ rights:

So if there was a risk that a person would be mistreated in Rwanda then they could not be sent there.

Human rights campaigners have highlighted the negative impact on refugee human rights, the cost of the scheme, and have questioned whether it will achieve its aims.

Enver Solomon, chief executive of the Refugee Council, said the proposal would not address the reasons why desperate people travel to the UK.

Amnesty International UK described the plan as “shockingly ill-conceived idea” that would waste public money.

Shadow culture secretary Lucy Powell said Labour advocates “boring measures” like curbs on people smugglers touting for business online.

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”.