Home Secretary Yvette Cooper has said small boat arrivals will be detained to be returned to France "in a matter of days".
Ms Cooper said people who cross the Channel will be taken into custody "immediately on arrival". A new one-in-one-out returns deal - the first agreement to return migrants to France since Brexit - will come into force on Wednesday.
Ms Cooper told Sky News: "The first step will be to be detaining people because people will be detained until they are removed to France. And we expect those detentions to start in a matter of days."
Describing the deal she said: "It will start with the numbers being lower and then we'll build up. But I think this is really important - the groundbreaking principle here is that people can be returned to France if they arrive on these dangerous and illegal small boats.
"In return, we will take people who have applied legally through a legal process and been through security checks. But we should be returning people who have made these illegal crossings."
But she refused to be drawn on the numbers who would be involved in the exchange deal. Ms Cooper said: "We don't want to provide operational information for criminal gangs who will then use it if they think there are particular numbers of people who are going to be detained on particular days. They will then use that and operate their boats and their gang operations around that, and that's not what we're prepared to tolerate."
She said: "I think the principle which says, frankly, if you arrive here on a small boat, you have paid thousands of pounds to a criminal people smuggler, you should be being returned. That money should be lost."
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