The full force of the law should be available against oil executives who are found to have fixed prices, David Cameron has indicated.
He said the Government was looking at ways of extending criminal law to cover alleged price-fixing activities in the energy sector.
The intervention came after the Commission carried out raids over claims that firms "colluded in reporting distorted prices to a price reporting agency to manipulate the published prices for a number of oil and biofuel products".
It suggested the alleged price fixing, which may have been going on since 2002, could have had a "huge impact" on the cost of fuel at the pumps.
Speaking to reporters on his trip to the US, Mr Cameron said: "The first thing is, these are hugely concerning allegations and if true very, very serious.
"The Office of Fair Trading (OFT) is involved in this investigation as I understand it but we have to get to the bottom of what happened first before I think we can pass judgment on the way regulators have worked in the UK.
"But it is hugely concerning. An investigation is under way."
Mr Cameron initially said that offences created in the Financial Services Act 2012, intended to prevent a repeat of the Libor scandal, could be applicable in this situation.
However, Downing Street later embarrassingly admitted he had been wrongly briefed and that was not the case. "It is totally unacceptable for firms to fix prices and force consumers to pay more," he said in a clarified quote.