Motorists may have been paying too much for their petrol because banks and other traders are likely to have tried to manipulate oil prices in the same way they rigged interest rates, an official report has warned. Concerns are growing about the reliability of oil prices, after a report for the G20 found the market is wide open to manipulation or distortion". Traders from banks, oil companies or hedge funds have an "incentive" to distort the market and are likely to try to report false prices, it said. Petrol retailers use oil price "benchmarks" to decide how much to pay for future supplies. The rate is calculated by data companies based on submissions from firms which trade oil on a daily basis - such as banks, hedge funds and energy companies. However, like Libor ... the market is unregulated and relies on the honesty of the firms to submit accurate data about all their trades. This is one of the major concerns raised in the G20 report, published last month by the International Organisation of Securities Commissions (IOSCO). In the study for global finance ministers, including George Osborne, the regulator warns that traders have opportunities to influence oil prices for their own profit. It points out that the whole market is "voluntary", meaning banks and energy companies can choose which trades to make public. IOSCO says this "creates opportunity for a trader to submit a partial picture in order to influence the [price] to the trader's advantage".