"Rosneft fixed and maintained high monopoly-like prices for gasoline, diesel fuel, air fuel and fuel oil in the summer of 2008, and also created discriminatory conditions and set different prices for one and the same commodity, which were neither economically nor technologically justified," the Federal Anti-Monopoly Service (FAS) said.
"The anti-monopoly service established that LUKoil abused its dominant position on the wholesale market of oil products in the summer of 2008 by fixing monopoly-like high prices for gasoline, diesel fuel, air fuel and fuel oil," the service said.
A FAS commission found it was no accident that LUKoil raised fuel prices when demand was highest.
"The actions by the oil company have given rise to social tension and economic problems in the country," FAS deputy head Pavel Subbotin said.
The commission also said the companies did not hurry to reduce prices on receiving recommendations to do so.
The anti-monopoly regulator sent its inquires to Russia's major crude producers LUKoil, Gazprom Neft (the oil producing arm of energy giant Gazprom), Surgutneftegaz, TNK-BP Management, Rosneft and other companies in July over hikes in fuel prices, and demanded in September that seven of them cut prices.
In late September, FAS also found actions by Gazprom Neft and TNK-BP to be in breach of competition laws, while allegations against Surgutneftegaz were dropped.
FAS head Igor Artemyev said the service would determine the size of the fines to be imposed on Rosneft, LUKoil, Gazprom Neft and TNK-BP within two weeks.