Norwegian company Statoil also confirmed its office in Stavanger was inspected by officials from the European Commission as motoring campaigners spoke of fears of consumers being conned.
The European Commission said in a statement 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".
The statement 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.
Price reporting agency Platts is also being investigated.
Tory MP Robert Halfon has led a campaign calling for a full investigation into alleged cartels and market manipulation in the oil market for the past three years. The Harlow MP, who founded the www.PetrolPromise.com website to campaign for cheaper petrol and diesel, said: "Last year, in a debate that I pressed for, Parliament voted unanimously for an investigation into the oil market. These latest allegations underline why that must happen urgently in the UK."
Quentin Willson, spokesman for FairFuelUK, said: "The sharp rises in fuel and oil prices we've seen recently have raised huge concerns about what is really driving them. The oil market isn't transparent or accountable enough. There is an urgent need for a powerful and independent watchdog to ensure that there's no market manipulation and that UK families and businesses pay a fair price for their fuel."
Peter Carroll, founder of FairFuelUK, said: "If it is proved that the oil market has been manipulated it will be a global scandal as damaging as the recent Libor debacle."