Two men are being detained by French police after four kilos of cocaine and cannabis were discovered hidden inside an official Vatican car with diplomatic number plates.

French radio network RTL said the vehicle, which belonged to Argentinian cardinal Jorge Mejia, had gone to be serviced a few days before it was stopped. The retired 91-year-old Librarian emeritus is currently bed-ridden following a heart attack.

Two Italian men, aged 30 and 41, are believed to have picked up the car and driven it to Spain where they allegedly collected four kilos of cocaine and between 150 and 200 grams of cannabis.

The pair then reportedly drove back to France with the haul stashed in the car, apparently convinced the diplomatic license plates would protect them from being searched at the border.

However, customs officials stopped the car during a routine check as it travelled through Chambéry, near the border with Switzerland and Italy, on Sunday, where they discovered the illegal stockpile.

Police in Lyon have opened an investigation into the incident.