A car bearing Vatican diplomatic plates was stopped in France with 4kg of cocaine and 200g of cannabis on board.

The car belongs to the 91-year-old Argentinian cardinal Jorge Mejia, emeritus librarian at the Holy See, who retired in 2003 and who is bedridden after a heart attack. Pope Francis, a fellow Argentinian, visited Mejia in hospital in Rome two days after being elected.

French radio reported that the cardinal's private secretary entrusted the vehicle to two Italian men to take it for its annual checkup. The two men drove to Spain to buy the drugs, thinking they would be protected by the diplomatic plates, according to RTL radio – a scenario not yet confirmed by legal sources.

The pair were picked up on Sunday at a toll station near Chambéry, in the French Alps, on their way back. Neither of the men had a Vatican diplomatic passport, so the Vatican was not directly implicated, French legal sources said.

The Vatican confirmed that the car had been stopped in France with the drugs on board and stressed that no staff were involved in the incident.