A family of three have become the first migrants to arrive in the UK under the so-called "one-in one-out" deal struck with France, a UK government official said on Wednesday.

"A family of three, including a small child, are the first to have arrived" under the deal, said the official, who asked to remain anonymous.

No further details were available, but the arrival follows the removal of four migrants from the UK to France, as part of the agreement aimed at deterring an unprecedented number of migrants making the perilous journey by boat from northern France to the UK's south coast.

Under the UK-France scheme, Britain can return small-boat migrants after they arrive across the Channel if they are deemed ineligible for asylum, including those who have passed through a "safe country" to reach UK shores.

In return, London will accept an equal number of migrants from France who are likely to have their asylum claim granted.

The interior ministry described the exchange as "critical first steps" following the announcement of the deal during a state visit to Britain by French President Emmanuel Macron in July.

"This is a clear message to people-smuggling gangs that illegal entry into the UK will not be tolerated," it said in a statement.

"We will continue to detain and remove those who arrive by small boat. And we will work with France to operate a legal route for an equal number of eligible migrants to come to the UK subject to security checks."

Tens of thousands of migrants have arrived annually on UK shores in recent years, fuelling domestic anger and the rise of Brexit figurehead Nigel Farage's hard-right Reform UK Party.

The journeys across one of the world's busiest shipping lanes have also repeatedly proved fatal for migrants. At least 23 people have died so far this year, according to a tally based on official French data.

Prime Minister Keir Starmer took power in July 2024 vowing to "smash the gangs" behind the journeys, and scrapped a costly scheme planned by the previous Conservative government to send some migrants to Rwanda.

The opposition Conservative Party has dismissed the "in-out" treaty as tinkering around the edges and unlikely to have the desired deterrent effect.

The exchange follows demonstrations outside hotels being used by the government to house migrants.

Locals in the town of Epping, northeast of London, took to the streets after an Ethiopian asylum seeker sexually assaulted a teenage girl and a woman in July.

The case ignited weeks of protests and counter-demonstrations, there and outside other migrant hotels. (AFP)