Seven of Italy's best-known olive oil companies are being investigated for allegedly conning consumers by passing off inferior quality olive oil as extra-virgin.
Extra-virgin oil costs, on average, about a third more than ordinary oil, netting the firms millions of pounds in extra profit.
Extra-virgin oil, which is typically green-gold in colour, is made by crushing the olives soon after they are picked and involves no chemicals or industrial refining.
Ordinary olive oil has a higher acidity and is produced with the aid of heat or chemicals. It is lighter in colour and has less flavour.
The alleged fraud was first discovered by an Italian consumer magazine in May and then investigated by the authorities. They announced this week that of 20 brands tested in the laboratory by specialists from the Italian customs agency, nine were found to be lower quality oil.
The producers caught up in the investigation include Bertolli, Santa Sabina, Primadonna - which is sold through the discount supermarket chain Lidl - and Antica Badia, which is sold through another cheap supermarket chain, Eurospin. The other three leading brands are Carapelli, Coricelli and Sasso.
The alleged fraud is now being investigated by prosecutors based in Turin. "The damage caused by this deceit is enormous, not just for consumers but also for the entire country and for the image of products that are made in Italy," said Rosario Trefiletti, the president of Federconsumatori, a consumer association. "It's a shameful business that requires prompt action by the authorities."
Maurizio Martina, the Italian agriculture minister, said it was important for the investigation to proceed in order to "protect consumers, as well as the thousands of honest olive oil producers" in Italy.
"For months now we have been increasing quality controls. In 2014 our inspectors carried out 6,000 checks and confiscated oil worth euros 10?million (pounds 7?million). It's vital to protect a sector as important as that of olive oil," she said.
Italy is the world's second largest producer of olive oil, after Spain.
The sector is worth an estimated euros 5?billion a year to Italy, with Puglia being the most productive region, followed by Calabria and Sicily.
Codacons, Italy's principal consumer association, said people who had bought the substandard oil should be entitled to compensation from the companies involved.
"We invite any family that bought oil from any of the firms caught up in the investigation to demand compensation of up to euros 5,000," said Carlo Rienzi, the association's president.
"If these offences are proven, the cost to consumers is enormous - not only the betrayal of trust but also the economic damage for people who have paid extra for an inferior product."
Coldiretti, an organisation representing Italian farmers, said consumers should be suspicious of oil purported to be extra-virgin that sold for just a few euros a bottle.
Italy has long-standing and seemingly endemic problems with fraud in its food and drink sector. Air-dried ham produced in Eastern Europe is passed off as Italian prosciutto and buffalo mozzarella has been found to be made from cow's milk rather than that of water buffaloes.
If the olive harvest is poor, oil is frequently supplemented with supplies from countries such as Greece, Tunisia and Morocco, but sold as 100 per cent Italian. In 2011 an investigation by customs officers and the Guardia di Finanza, the fraud squad, found that four out of five bottles of "Italian" olive oil contained low-quality oil from other Mediterranean countries.
"It's a very widespread problem and it is on the increase," said Elga Baviera, a biologist and expert on food safety.
"The food production business is full of scams, notwithstanding the efforts of authorities to contain the phenomenon. It's a business worth billions."