LONDON - Britain said it would begin implementing a deal to return migrants who arrive on small boats to France within days after a treaty on the arrangement - a key part of British plans to cut illegal migration - is ratified on Tuesday.

Prime Minister Keir Starmer and French President Emmanuel Macron announced the "one in, one out" pilot scheme on migrant returns last month.

Under the new deal, France has agreed to accept the return of undocumented people arriving in Britain by small boats, in exchange for Britain agreeing to accept an equal number of legitimate asylum seekers with British family connections.

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A treaty on the scheme was signed last week but not previously announced ahead of Tuesday's ratification. Britain said the European Commission and EU member states had given the green light to the plan.

Starmer, whose popularity has fallen since winning an election landslide last year, is facing pressure to stop small boat arrivals from the populist Reform UK party, led by Brexit campaigner Nigel Farage.

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Britain's interior ministry said it expected detentions to begin within days.

"This is an important step towards undermining the business model of the organized crime gangs that are behind these crossings," British interior minister Yvette Cooper said.

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Under the agreement with France, government sources previously said they were looking at about 50 returns a week, or 2,600 a year, a fraction of the more than 35,000 arrivals reported last year, though the scheme could be scaled up.

More than 25,000 people have arrived on small boats so far in 2025, and the government has targeted people smugglers with sanctions, clamped down on social media adverts and is working with delivery firms to tackle the illegal work that is often promised to migrants.