A "one-in, one-out" deal in which Britain can return some migrants who cross the Channel in small boats back to France came into force on Tuesday, the U.K. government announced.

The interior ministry said that British authorities were "operationally ready" and that migrant detentions were expected to begin "within days" following ratification of the agreement.

"Today we send a clear message — if you come here illegally on a small boat you will face being sent back to France," Prime Minister Keir Starmer said in a statement after striking the deal with French President Emmanuel Macron last month.