LONDON – British border authorities have detained the first group of migrants under a pilot plan that will send some who cross the English Channel on small boats back to France.

The migrants were detained Wednesday, the day the program came into force, and will be held at immigration removal centers until they are returned to France, the Home Office said on Thursday.

“That sends a message to every migrant currently thinking of paying organized crime gangs to go to the U.K. that they will be risking their lives and throwing away their money if they get into a small boat,” Home Secretary Yvette Cooper said in the statement.

Prime Minister Keir Starmer and French President Emmanuel Macron announced the deal last month as the U.K. government struggles to tamp down criticism that it has lost control of the country’s borders. While the pilot involves a limited number of people, U.K. officials suggest the deal is a major breakthrough because it sets a precedent that migrants who reach Britain illegally can be returned to France.

Critics say the program will do little to deter migrants because the numbers returned to France are small and loopholes in the treaty will allow many people who enter Britain illegally to remain in the country as they pursue human rights claims.