The local prefecture said in a statement: “Rescuers found that the boat, which was heavily loaded, was in difficulty and that some of the people were in the water.
“Rescuers began to recover the people in difficulty.
“At the same time, further searches were carried out to find people who could be stranded at sea.”
It was then that the baby was discovered, the prefecture added. Prosecutors have opened an investigation into the death.
2024 is already the deadliest year for migrant crossings of the English Channel since 2018.
On 3 September, six children and a pregnant woman were among 12 people who died after a boat carrying dozens of people sank off the French coast.
A month later, four people, including a two-year-old boy, died after seemingly being “trampled to death” on two separate boats.
The latest sinking brings the total number of deaths from migrants attempting to cross the Channel this year to at least 53.
According to Home Office figures, more than 26,000 migrants have arrived in Britain on small boats since the start of the year.
The International Organisation for Migration (IOM) has said deaths in the Channel are “preventable”.