EQUINOR ASA 
Oil firm Eni said the European Commission had asked it to provide information, although it was not being probed.
The Italian company's statement came a day after the offices of Shell, BP and Statoil were raided by investigators over suspected oil price manipulation.
The surprise searches of major oil companies, but not their competitors the powerful privately-owned trading companies, were one of the biggest cross-border probes since the Libor scandal and sent shock waves through the secretive industry.
Authorities have sharpened scrutiny of financial benchmarks around the world since slapping large fines on some of the world's biggest banks for rigging interest rate benchmarks.
"If it turns out to be the case that hard-pressed motorists and consumers have been hit in the pocket by manipulation in the market, the full force of the law should be down upon them. There is no doubt about that," Britain's Energy Secretary Ed Davey told parliament.
A spokesman for Prime Minister David Cameron said he expected companies to fully comply with the investigation.
London is home to some of the biggest trading desks in the oil business. Following the Libor scandal, Britain approved legislation making a criminal offence of false or misleading statements in relation to the setting of financial benchmarks.
"The investigation's focus on potential collusion over price reporting draws parallels with the investigations by European and U.S. regulators into Libor rates," Fitch ratings said.
The Commission has said it had concerns that companies may have colluded in reporting distorted prices to a price reporting agency to manipulate prices for oil and biofuel products.
It also said companies may have prevented others from participating in the price assessment process of a pricing agency, which it did not name.
Platts, the world's largest pricing agency and a unit of U.S. McGraw Hill, is cooperating with the probe. On Wednesday, Antoine Colombani, the Commission's spokesman on competition policy, said only one pricing agency was involved in the probe.
"Even small distortions of assessed prices can have an impact on final prices, potentially harming consumers," he said.
FINES AND REPUTATION
Fitch said that in the event of large fines, oil company balance sheets would likely absorb the blow thanks to big cash reserves.
The biggest settlement so far over Libor was the 1.4 billion Swiss francs that UBS agreed to pay to U.S., British and Swiss regulators.
"Other than fines, if an oil company is found to have distorted prices, it could face longer-term risks from damage to reputation," said Fitch, noting that BP was excluded from some business in the Gulf of Mexico following the Macondo spill.
Sources and officials at major trading houses Glencore, Vitol, Trafigura, Gunvor and Mercuria said they have not been asked so far to cooperate with the probe.
Swiss antitrust authority Weko said there were no proceedings underway as part of the EU prove. Switzerland is home to most global oil trading houses.
French oil major Total also said there had been no inspections at its offices. Last year it wrote to regulators to question the way oil prices were determined, bringing into the spotlight the pricing mechanisms of price reporting agencies.
The methodology designed by Platts for daily assessments on the physical oil markets is used to close deals worth at least $2.5 trillion a year.
Critics say the system is only a snapshot of the market and excludes most deals, making it vulnerable to manipulations.
Supporters say it is the best mechanism so far devised to assess huge but often opaque markets.
Olivier Jakob from energy consultancy Petromatrix said he believed the current pricing system was far from perfect but Platts still remained the main reference for the entire market.
"They are essential... So if you were to close Platts tomorrow, you would have a very big problem," he said.
Leen Broekhuizen, partner at energy consultancy SunGard, said the probe could result in the EU regulating oil trading more strictly after having already put in place measure to prevent market abuse in the physical gas and power markets.
(Additional reporting by Stephen Jewkes, Emma Farge; Writing by Dmitry Zhdannikov; Editing by William Hardy)
By William James and Dmitry Zhdannikov
