At least four people have died as they were trying to get onboard an inflatable boat to attempt the perilous sea crossing from northern France to the UK.
The prefect of the Pas-de-Calais region in northern France, François-Xavier Lauch, said 38 others were rescued on Thursday, including one whose condition involved medical emergency.
He spoke while rescue operations were still ongoing on Equihen Beach on Thursday morning.
Lauch said migrants were carried away by dangerous currents as they were trying to embark on a "taxi-boat", the name authorities use for small motorised boats, usually inflatable, used by traffickers to pick up people along large stretches of the northern French coast.
Thursday's incident happened along a broad expanse of sand, backed by dunes and a forest where people attempting the perilous crossing hide out, sometimes for days at a time, as they wait for boats and suitable weather and sea conditions.
Police patrol on buggies and keep watch from the remains of World War II-era bunkers but cannot prevent all departures on a beach so long.
Attempted crossings and deaths have surged in recent days.
French maritime authorities said on Wednesday that 102 people had been rescued in two separate operations while trying to cross the channel.
Last week, two people died in a similar incident off the coast north of Calais.
Campaign groups for migrant rights have long warned that increasingly vigorous efforts by French police to prevent boat departures from beaches, including using knives to hack and puncture inflatable boats to render them unusable, are encouraging the use of "taxi boats", which increases the risks of drownings, injuries and the need for rescues.