Preliminary data from local authorities showed that 38 others were rescued, with one person being treated for hypothermia, the French news channel BFMTV reported.
The boat went down well offshore near Boulogne-sur-Mer in the northern French region of Hauts-de-France while attempting to cross the channel from France, local authorities told BFMTV.
A large-scale emergency response has been deployed to the scene.
Authorities blamed smugglers for the tragedy, and the victims' nationalities remain unknown, AFP reported.