Sir Keir Starmer and Emmanuel Macron's migrant deal comes into force today, with detentions set to begin from tomorrow.
The "one in, one out" pilot scheme - which allows the UK to send some people who have crossed the Channel back to France in exchange for asylum seekers with ties to Britain - was signed last week, and has now been approved by the European Commission.
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Home Secretary Yvette Cooper told Sky News Breakfast with Kamali Melbourne that this will apply to new entries, rather than those already here, who will be "detained immediately upon arrival".
She declined to say how many people could ultimately be sent back, claiming that would provide operational information to people smuggling gangs who would work around the numbers.
There have been reports that under the scheme only 50 people a week will be returned to France.
In exchange for those being returned, the same number of people will be able to come to the UK on a new legal route, provided they have not attempted a crossing before and subject to stringent documentation and security checks.
Ms Cooper stressed it was a pilot scheme, saying the numbers "will start lower and build up".
"Part of what we're doing here is trialling the concept," she said.
"The principle says, frankly, if you arrive here on a small boat, you have paid thousands of pounds to a criminal people smuggler actually, you should be being returned, that money should be lost... and we should be taking those who have applied lawfully who've been through security checks."
It comes as 2025 is on course to be a record year for crossings.
Approximately 25,436 people have already made the journey this year, according to PA news agency analysis of Home Office figures - 49% higher than at the same point in 2024.
The prime minister and the French president hailed the deal as a "good agreement" when it was first announced during the latter's visit to the UK last month.
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Sky News political correspondent Rob Powell said while it was a "policy win" for the government, the numbers must eventually "go a lot higher" than 50 per week if it is to work as a deterrent.
"The average crossing rate is about 800 a week, so this will need to go up by a sizeable factor for that message to start seeping through to people trying to make that crossing," Powell said.
The aim will be to make asylum seekers believe the "risk of going back to France is so big that they shouldn't bother parting with their cash and paying smugglers" to make the crossing.
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The scheme is set to run until June 2026, pending a longer-term agreement.
Asked if this means the French could pull out next year, Ms Cooper said: "This is a joint agreement. It'll be reviewed month by month, and we'll make adjustments to it month by month as well."
The Conservatives have branded the agreement a "surrender deal" and said it will make "no difference whatsoever".
(c) Sky News 2025: UK-France migrant returns deal comes into force