A "one-in, one-out" deal in which Britain can return some migrants who cross the Channel in small boats back to France will come into force on Wednesday, the UK government has announced.

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

The agreement seeks to curb record levels of irregular Channel crossings, which are causing discontent in Britain and helping fuel the rise of the hard-right Reform UK party.

Under the arrangement – for now a pilot scheme set to run until June 2026 – irregular migrants arriving on UK shores could be detained and then returned to France if they are deemed ineligible for asylum.

This would include those who have passed through a "safe country" to reach the UK, according to a Home Office fact sheet.

In exchange, the UK will accept an equal number of migrants from France who can apply for a visa to enter the UK via an online platform, giving priority to nationalities most vulnerable to smugglers and people with ties in Britain.

If approved, they will have a three-month period in which they can enter the UK and apply for asylum.

The text of the treaty published and laid in front of parliament on Tuesday stipulates that Britain will submit a request for transferring a person back to France within 14 days of their arrival in Britain.

The whole process of returning someone could take three months and the UK will cover all the costs until the migrant is handed over. Unaccompanied minors will not be eligible for deportation under the scheme, the treaty adds.

The two governments signed the final text last week, with the European Commission giving its "green light on this innovative approach", Britain's Home Office said in a statement late on Monday.

French Interior Minister Bruno Retailleau also signalled French agreement, writing on X: "I reaffirm France's determination to stop the flow and save lives."

Refugee charities have criticised the deal, urging the British government to provide more safe and legal routes for asylum seekers instead.

The number of migrants making the dangerous journey in flimsy dinghies this year crossed 25,000 at the end of July, the highest tally ever at this point in the year.

Speaking on the Hong Kong Today programme, RTHK's UK correspondent Gavin Grey said many people in the UK remain sceptical about the plan, which some believe is ripe for abuse.

"Many people in this country believe we are simply allowing in too many people that we do not know of their background – whether they're criminals, whether they're going to commit crimes when they're here – and it is causing a very, very tense situation at some of these protests that are going on," he noted. (AFP/ RTHK)