The 24-year-old, who has not been named, is thought to have drowned in the early hours of Tuesday morning.
“Another migrant found her and alerted the authorities. Prosecutors in Boulogne have established that she was a 24-year-old Eritrean who was hoping to claim asylum in the United Kingdom. They have opened a criminal investigation.
“Medical examinations are still being carried out, but she undoubtedly died from drowning after falling off a boat.
“A request for assistance has been sent to the British authorities, to find witnesses on board the boat she was on. It is believed to have reached the United Kingdom.”
The woman is the latest fatal casualty on the extremely dangerous crossing between northern France and Britain.
People smugglers organise the journeys, charging the equivalent of up to £1000-a-time.
According to the French authorities, there was no sinking recorded in the English Channel overnight from Monday to Tuesday.
But two successful crossings were reported – one at 1.45am with 50 migrants on board the boat, and another at 2.39am, with 40 migrants on board.
There were also three attempted crossings – one on the beach at the hamlet of Hemmes-les-Marck, which resulted in the arrest of a migrant and a German national for transporting nautical equipment.
In August, six migrants died after a boat carrying mainly Afghans sank on the route.
And in November 2021, at least 27 migrants died after their dinghy collapsed below them.
More than 21,000 people have arrived in the United Kingdom via the English Channel since the start of 2023.
August saw a monthly record established, with more than 5,300 people arriving on around 100 small boats.
A sizeable number come from Eritrea, the east African country with one of the worst human rights records in the world.
Around 5000 refugees flee the country every month, complaining of enslavement, torture and the threat of murder, as well as malnourishment and economic destitution.
Despite tougher legislation, which now prohibits migrants who have arrived illegally on British soil from seeking asylum, the British government is struggling to stop the small boats, with numbers exploding every month.
It comes as UK Home Secretary Suella Braverman claims cross-Channel migrants should not be seen as refugees.
She is due to give a keynote speech in Washington D.C. on Tuesday, saying: “Nobody entering the UK by boat from France is fleeing imminent peril.”