The surprise load was uncovered when the cargo ship, which normally carries stages of Europe's Ariane, Soyuz and Vega rockets, was preparing to depart the Pariacabo port on August 5, destined for France, Samuel Finielz, prosecutor of French Guiana's capital Cayenne, said late Thursday.

As the cargo was being manoeuvred, a container fixed discreetly to the vessel's hull fell into the water.

"There was just under eight kilos of cocaine inside," Finielz told AFP.

A source close to the investigation described the incident as "not at all usual", and said the cargo of merchant vessels -- unlike that of cruise ships -- was usually very well monitored.

"It is hard to comprehend how drugs could have been found in what is meant to be a secure place: the port of Pariacabo, which is attached to the Guiana space centre," added the source.

France's CNES space agency and commercial launch company Arianespace have not responded to requests for comment.

The European Space Agency (ESA), CNES and Arianespace conduct launches from Kourou which has been operational since the 1960s.