An official Vatican car with diplomatic licence plates has been found riddled with several kilos of cocaine and cannabis in the French Alps, according to local reports.

The car belongs to an Argentine cardinal, 91-year-old Jorge Maria Mejia, who is also emeritus librarian at the Holy See. Mejia retired in 2003 and is confined to bed following an heart attack. Pope Francis visited Mejia just two days after being elected.

According to French newspaper Le Monde, the cardinal's personal secretary entrusted two Italian men, aged 31 and 41, with taking the car for its annual checkup.

They drove the vehicle to Spain to buy four kilos of cocaine and 200 grams of cannabis and returned to France, according to reports, in the belief that the diplomatic plates would protect them.

But in Chambery, near the border with Switzerland and Italy, the two were stopped by customs officers for a routine check. After the bizarre discovery, they were arrested and taken into custody. Judicial police in Lyon have opened an investigation for drug trafficking.

The Vatican confirmed the car had been stopped in France with the drugs on board. Since neither of the men had Vatican diplomatic passports, the Vatican is not legally implicated, French sources say.