Oil giants Shell and BP are being investigated over allegations of price-fixing, it emerged tonight.
If the firms are found guilty of manipulating the market, it would mean consumers have been paying over the odds for years.
The European Commission said a series of unannounced inspections were carried out by its officials on oil companies earlier today.
A spokesman for BP said: "BP is one of the companies that is subject to an investigation that was announced earlier today by the European Commission.
"We are co-operating fully with the investigation and unable to comment further at this time."
Shell said its offices in London and Rotterdam were visited, as was Statoil's Stavanger office.
A statement from the European Commission said: "The European Commission can confirm that, on 14 May 2013, Commission officials carried out unannounced inspections at the premises of several companies active in and providing services to the crude oil, refined oil products and biofuels sectors.
"The commission has concerns that the companies may have colluded in reporting distorted prices to a Price Reporting Agency to manipulate the published prices for a number of oil and biofuel products.
"Furthermore, the commission has concerns that the companies may have prevented others from participating in the price assessment process, with a view to distorting published prices.
"Any such behaviour, if established, may amount to violations of European antitrust rules that prohibit cartels and restrictive business practices and abuses of a dominant market position (Articles 101 and 102 of the Treaty on the Functioning of the EU and Articles 53 and 54 of the EEA Agreement)."
The European Commission statement added that even small distortions of assessed prices could have a "huge impact on the prices of crude oil, refined oil products and biofuels purchases and sales, potentially harming final consumers".
It said that unannounced inspections are a preliminary step to investigating suspected anti-competitive practices and do not mean that the companies are guilty of any wrongdoing.
There is no legal deadline to complete inquiries.
RAC technical director David Bizley said the allegations were "worrying news for motorists" who are already suffering due to the high cost of keeping a vehicle.
He added: "The Office of Fair Trading inquiry concluded at the end of January that the UK fuel market was operating fairly and not against the best interests of motorists, and therefore that a Competition Commission investigation was not needed.
"Motorists will be very interested to see what comes of these raids. Whatever happens the RAC will continue to campaign for greater transparency in the UK fuel market and for a further reduction in fuel duty to stimulate economic growth. "