The French government sent a written response to Britain's request to conduct joint patrols on French soil, in order to prevent migrants from sailing to the United Kingdom.
This relates to a proposal made by British Prime Minister Boris Johnson during a telephone conversation with French President Emmanuel Macron on November 25, in the wake of the sinking of a ship in the English Channel, which killed 27 people.
Johnson sparked outrage in France when he published his proposal to Macron.
In this regard, French Prime Minister Jean Castix sent a letter to his British counterpart.
"We have always accepted the study and discussion of British proposals to enhance cooperation in good faith. We have accepted some, and we have rejected others," Castix wrote in the letter.
"We cannot accept, for example, that British police or soldiers patrol our coasts, as this detracts from our sovereignty," he added.
A source from the French government said that "returning migrants is not an option for us, it is not a serious or responsible way to deal with this issue."
In his message, Jean Castix indicated that "more than 700 police and gendarmes survey the Opal coast (north) daily, to prevent dilapidated boats from sailing" towards England.
Castix reminded the British Prime Minister that "some of these operations are carried out with a financial contribution from your government, according to the cooperation agreements concluded between us."
"However, these efforts only contain the phenomenon, and not provide a permanent solution," the French prime minister added, calling on Britain to open "legal immigration avenues for those who have legitimate reasons to seek" to this country.
For its part, the UK government pledged to work with France in "close cooperation and partnership" in the wake of the Castex letter.