A group of alleged drug mules travelled on Costa Cruises ships between the Mediterranean and Brazil.

But ships’ crew suspected something was wrong when young French-speaking couples appeared among elderly passengers on the transatlantic voyages.

Their tip-offs led to a police raid on the Costa Fascinosa in mid-voyage off the Canary Islands and the arrest of 31 people, most of whom went on trial in Marseilles yesterday.

The eight women and 23 men were mainly alleged mules in so-called ‘cocaine cruises’.

The couriers, in their twenties, were recruited in Nice to carry cheap Moroccan cannabis resin to Rio de Janeiro and other ports. There it was exchanged for cocaine that they took back to Europe at a total profit of tens of millions of pounds.

Chief judge Patrick Ardid said at the opening of the trial: “This is new, a first. We are judging an international trade among three continents and the use of cruise ships.”

The leaders were “audacious innovators who had the ability to create new routes for cocaine”, he said.

The ring was rounded up in March 2014 thanks to observant crew, he said.

“They wondered what these young people were doing going on cruises with old people when they were not there to accompany their grandmothers.”

Four of the accused, including Karim Moutakhaouil, said to be a ringleader, are on the run and are being tried in their absence.

After seizing 25kg of Brazilian cocaine in the cabins of Costa Fascinosa, destined for Venice and Dubrovnik, prosecutors found a well-organised operation, The Times reported.

Youngsters, mainly from Les Moulins, an immigrant district of Nice, were given cruise tickets and promised up to €10,000 for making the return trip.

The smugglers knew that cruise passengers were not searched.

The young couples boarded ships, often in the Italian port of Savona and picked up cannabis resin at a stop in Casablanca.

Packets were strapped to their bodies in a hotel room while other passengers were sightseeing.

They avoided other mules on board, which also drew suspicion from crew, especially when they were seen getting into taxis together in Brazil.

The cars took them to a spot where the cannabis was swapped for cocaine.

Police calculated that the gang had made $76,000 on each kilo of cocaine brought to Europe. The margin was boosted by the high value of their cannabis, which cost $450 but was worth $15,000 to Brazilian traffickers.

In two years cruise ships were used 13 times, with up to a dozen mules on each vessel. Some stopped over in Caribbean ports, according to the investigation.

The operation was allegedly run out of Morocco by Moutakhaouil, 35, and Victor Sanchez, a Spanish-French national, who denies the charges.

One mule told investigators that cocaine had been strapped to him at a stop on the island of St Marten after a return cruise from Miami. He was paid only €3,000 for four cruises and his “employers” kept promising to settle their debt after one more cruise, he said.

The trial continues.