Four migrants died overnight and a fifth was in critical condition on Sunday after trying to reach Britain from northern France in freezing temperatures, the French maritime authority said.
About 70 migrants, including small children, were rescued, said a source familiar with the situation.
The fatalities were the first reported migrant deaths on the Channel in 2024.
The group was attempting to reach a vessel off the resort town of Wimereux when their small boat got into difficulty around two am (0100 GMT), the maritime prefecture said.
"We have four dead migrants and one migrant in a critical condition at the hospital in Boulogne-sur-Mer," a spokeswoman for the maritime prefecture told AFP.
The crew of a French tow vessel, the Abeille Normandie, went to the rescue and spotted "unconscious and lifeless people" in the water, the official said, estimating the water temperature to be around nine degrees Celsius.
The survivors were taken to Calais.
According to one person, who spoke on condition of anonymity, around 70 migrants were brought in at around three am, including "entire families with children, some of them very young."
"Some of the survivors did not stay and told us they wanted to go to the Dunkirk train station to reach an accommodation centre in Armentieres," the person added.
'Hypothermia or drowning'
Jean-Claude Lenoir, head of the Salam association, said migrants took huge risks by trying to board bigger vessels in the water in the current conditions.
"Migrants want to get on board at all costs," he told AFP. "They quickly fall victim to hypothermia or drowning."
In December, two migrants died in two separate incidents as they attempted to cross the Channel.
The region around Calais, the jumping-off point for the shortest crossing to England, has long been a magnet for migrants.
More than two decades after the closure of a Red Cross center in Sangatte, hundreds of people still live in tents and makeshift shelters near Calais and Dunkirk, hoping for a chance to make the crossing hidden in a truck or aboard a small boat.
The boats are a political priority for the British government and a bone of contention with France, as tens of thousands of people a year have been making the dangerous crossing.
According to the British government, nearly 30,000 migrants crossed the Channel from mainland Europe to Britain in small boats in 2023, an annual drop of more than a third.
French authorities say that boats are increasingly overloaded.
In November 2021, at least 27 people drowned when their dinghy capsized.
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