French fraud investigators say they have uncovered a €6m (£5m) scam in which Italian kiwis were passed off as more expensive French-grown fruit.

Consumer officials say 15,000 tonnes of the fruit were wrongly labelled over three years and have launched legal action against seven suppliers, who risk prison sentences and €300,000 fines if convicted.

Suspicions were raised when French kiwi producers queried the amount of fruit on the market. They produce about 45,000 tonnes of the fruit annually, compared with 400,000 tonnes in Italy.

Investigators said they discovered 12% of kiwis stamped “French origin” had been imported from neighbouring Italy. With French kiwis selling at €0.70 each and Italian kiwis at €0.50, fraudsters were allegedly making €0.20 on each fruit sold, netting a profit of an estimated €6m.

Le Parisien newspaper, which called the scandal “kiwigate”, said the price difference was the result of cheaper labour and production costs in Italy, where it also claimed fruit was treated with products banned in France, enabling a higher yield.

Kiwis are among the 10 most popular fruits in France, with an average 2.8kg consumed every year per person. France has 1,200 kiwi producers and most of the crop is grown in the Midi-Pyrénées and Aquitaine regions. Producers claim that at the end of the kiwi season, around March and April, when fewer French kiwis are available, the market is being flooded with Italian kiwis, artificially lowering prices.

It was the sudden availability of French kiwis, at a time they were supposed to be out of season, that alerted the authorities.

“We said to ourselves, such large volumes at the end of season aren’t possible,” Adeline Gachein, the director of the kiwi producers national office, told Le Parisien.

Investigators said international cooperation under the network Food Fraud, created after the scandal of horsemeat passed off as beef, helped uncover the alleged scam.