LONDON -- The first illegal migrants have been detained under the new return deal between Britain and France, the British government confirmed on Thursday.

"We have detained the first illegal migrants under our new deal before returning them to France," British Prime Minister Keir Starmer said in a post on X.

"If you break the law to enter this country, you will face being sent back," he added.

The Home Office said the migrants were detained after arriving on a small boat on Wednesday and that they will now be returned to France in the coming weeks.

Home Secretary Yvette Cooper did not confirm how many migrants were being held or will be returned to France. Critics have raised doubts whether the new deal will create any deterrent to potential migrants.

"The transfers to immigration removal centers are under way as we speak, so we won't provide operational details at this point that criminal gangs can simply use and exploit," Cooper said.

"But no-one should be in any doubt: anyone who arrives from now on is eligible for immediate detention and return," she added.

British officials are expected to make referrals for returns to France within three days of a migrant's arrival by small boat, with French authorities responding within 14 days.

The pilot scheme, set to last 11 months. will see Britain accepting an equal number of asylum seekers from France who have not tried to cross illegally and can pass security and eligibility checks.

As of 30 July, more than 25,000 people had crossed the Channel in small boats in 2025, about 49 percent higher than at the same point in 2024, according to Home Office data.