A seven-year-old girl drowned on Sunday when a small boat carrying 16 migrants heading from northern France to Britain capsized, the prefecture in France's Nord department said.
The boat "was not appropriately sized to carry so many people," the local authority said in a statement, causing it to capsize soon after people boarded in the Aa canal a few kilometres from the waterway's exit into the Channel.
The inland accident is the latest in a trend that has seen migrants aiming to get to Britain from France boarding boats away from the coast to avoid stepped-up surveillance there.
Police and firefighters rushed to the scene after a person out for a walk raised the alarm, the prefecture added.
The seven-year-old girl had died "on the spot of the effects of cardio-respiratory arrest," prosecutors in Dunkirk told AFP, adding that rescuers' attempts to resuscitate her had been "in vain".
"Several people are in custody" over the incident, they added, with investigators probing possible charges including manslaughter, people-smuggling and forming a criminal gang.
The prefecture said the girl's parents, who were travelling with three more of their children, were taken to a hospital in Dunkirk.
Prosecutors said that 10 children aged seven to 13 had been on the boat, with the prefecture identifying the other passengers as the girl's parents, another couple and two young men.
It added that the vessel was "apparently stolen".
Migrant crossings of the English Channel remain a sore point between Britain and France, with 671 reaching England in February.
A deal struck in March 2023 provides for 541 million euros ($586 million) in British funding for hundreds of French police to prevent migrants from putting to sea.
British interior minister James Cleverly told AFP in January that the two countries would "expand upon" cooperation he said had proved successful, citing figures showing a 36-percent reduction in crossings in 2023.
'Killed again'
"Border policy has killed again today," French migrant aid group L'Auberge des Migrants wrote on X (formerly Twitter).
Sunday marks the third fatal incident in a Channel crossing attempt this year.
A 22-year-old Turkish man died on Wednesday after falling overboard into the sea off Calais, while two more migrants remain missing.
An Eritrean citizen suspected to be part of a gang has been charged with manslaughter in that case and held in custody since Saturday.
Five more people including a 14-year-old Syrian boy were killed in mid-January as they tried to board a boat in chilly seas south of port city Calais.
Last year, 12 people were killed in crossing attempts, while 29,437 reached Britain, according to figures from London's interior ministry.
Around 20% were from Afghanistan, followed by contingents from Iran, Turkey, Eritrea and Iraq.
A major people-smuggling network organising Channel crossings in small boats was dismantled in an international operation on February 21, with 19 people arrested in Germany.