The seizure by two navy ships on Monday morning followed a tip-off by anti-drug and maritime intelligence authorities and British police, authorities in France's northwestern Atlantic Maritime region said.

The six-person crew of the 20-metre-long (66-foot) ship, which sank in the wake of the raid, are set to be handed over to the French courts, said Stephane Kellenberger, the public prosecutor in the port city of Brest.

The operation was part of France's Corymbe naval mission, which has been deployed in the Gulf of Guinea since 1990 to ensure security in an area where piracy is common.

The authorities did not reveal the nationality of the boat's crew.

"In total, 9.6 tonnes of cocaine with a market value of nearly 519 million euros ($609 million) was seized from a boat," the Atlantic Maritime prefecture said, adding that the vessel was not registered in any country and was likely bound for Europe.

Since the beginning of the year, the French navy has seized a total of 54 tonnes of cocaine in the Atlantic and Pacific oceans.

That compares with 47 tonnes in 2024 and 20.2 tonnes in 2022, according to the maritime authorities.