A bus driver who failed a drugs test launched an investigation into a herbal tea that has stunned Italian authorities.

Cafes and food stores in Italy have been unwittingly selling herbal tea containing cocaine for years.

Italian authorities made the shock discovery after a bus driver tested positive for the illegal stimulant in a routine drugs test, according to Italian newspaper La Repubblica.

The 38-year-old, who had a 10-year spotless employment record at the Genovese transport authority where he worked, was flabbergasted at his test result and denied any wrongdoing on his part to the company doctor.

The man said the only feasible explanation was that he had brewed a cup of "delisse alla coca" herbal tea with two teabags the day before, which he bought from a food store in central Genoa that he frequently patronised.

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The driver, referred to only as Roberto, claimed he enjoyed the brew as it made him feel more alert at the wheel.

To verify the story, the doctor told Roberto to bring in two teabags so he could try the intriguing brew himself.

Upon also testing positive for cocaine use the next day, the doctor notified the Italian food authorities, which launched an investigation, reported news site The Local.

Analyses showed that delisse alla coca was made with small quantities of cocaine leaves and contained sufficient amounts of the active ingredient of the drug, cocaine hydrochloride, that would be strong enough to impair the judgement of motorists.

There was no evidence of suspicious activity in the shop's records, which showed it had been legitimately purchasing the herbal tea bags from a Milan-based Peruvian wholesaler for years.

The case has stunned authorities for being undetected by Italian customs controls for years.

A removal order for the tea on all Italian shops has been launched. The sole Peruvian importer is being investigated.