Oil giants Shell and BP are being investigated for possible price-fixing, raising concerns road users may have been paying too much for fuel.
The European Commission is conducting the probe over fears companies may have distorted figures to a price reporting agency.
Motoring campaigners have spoken out about fears the public have been conned, with unannounced inspections taking place at the oil companies' offices (see our Cheap Petrol & Diesel guide to keep costs down).
The European Commission says even a small distortion of assessed prices could have a "huge impact on the prices of crude oil, refined oil products and biofuels purchases and sales".
MP Robert Halfon says: "High oil prices are crushing families across Britain. Motorists are being taken for a very expensive ride, now oil companies must come clean and show some responsibility for what is happening to the international oil price."
The European Commission says the inspections are a preliminary step and does not mean the companies are guilty of any wrong-doing.
There is no legal deadline to complete investigations, it added.