Prison guards have discovered 15 cannabis plants growing amongst the tomatoes in garden plots used by inmates at Saint-Martin-de-Ré prison in western France.

The plants were found to have reached rapid maturity thanks to the recent heatwave which has seen temperatures soar above 40 degrees Celsius, according to a report in the local Sud Ouest daily.

The plants had grown to a height of 80 centimetres and it is reported that the "suspicious colour of the foliage" aroused the curiousity of prison supervisors during a routine inspection walk.

The La Rochelle prosecutor has been informed and an investigation has been launched into the case.

The find is just the latest in a series of drugs-related incidents at the prison and the authorities have promised to increase surveillance after it was concluded that the plants will have been smuggled in by friends or family on the outside.

Despite reports indicating that cannabis smoking has a calming effect on violence in the prison, it is feared that a formalized trade in drugs will have the opposite effect as rival groups battle for control, the newspaper reported.