The London offices of BP and Shell have been raided by European regulators investigating allegations they have "colluded" to rig oil prices for more than a decade.

The European commission said its officers carried out "unannounced inspections" at several oil companies in London, the Netherlands and Norway to investigate claims they may have "colluded in reporting distorted prices to a price reporting agency [PRA] to manipulate the published prices for a number of oil and biofuel products".

The commission said the alleged price collusion, which may have been going on since 2002, could have had a "huge impact" on the price of petrol at the pumps "potentially harming final consumers".

Lord Oakeshott, former Liberal Democrat Treasury spokesman, said the alleged rigging of oil prices was "as serious as rigging Libor" – which led to banks being fined hundreds of millions of pounds.

He demanded to know why the UK authorities had not taken action earlier and said he would ask questions of the British regulator in Parliament. "Why have we had to wait for Brussels to find out if British oil giants are ripping off British consumers?" he said. "The price of energy ripples right through our economy and really matters to every business and families."

RAC technical director David Bizley said the allegations were "worrying news for motorists" who are already suffering due to the high cost of keeping a vehicle.

"Motorists will be very interested to see what comes of these raids. Whatever happens the RAC will continue to campaign for greater transparency in the UK fuel market and for a further reduction in fuel duty to stimulate economic growth."

Four months ago the Office of Fair Trading (OFT) ruled out an investigation into petrol price fixing after finding "very limited evidence" that pump prices rise quickly when the wholesale price goes up but fall more slowly when it drops.

The European authorities declined to name any of the companies raided but BP, Shell, Norway's Statoil and Platts, the world's leading oil price reporting agency, all confirmed they are being investigated.

In a statement Shell said: "We can confirm that Shell companies are currently assisting the European commission in an inquiry into trading activities."

BP said: "BP is one of the companies that is subject to an investigation that was announced by the European commission. We are co-operating fully with the investigation and unable to comment further at this time."

Statoil, which is 67%-owned by the Norwegian government, said: "The authorities suspect participation by several companies, including Statoil, in anti-competitive agreements and/or concerted practices contrary to Article 53 of the European Economic Area (EEA) [market manipulation].

"The suspected violations are related to the Platts' Market-On-Close (MOC) price assessment process, used to report prices in particular for crude oil, refined oil products and biofuels, and may have been ongoing since 2002."

Platts said the investigators had "undertaken a review at its premises in London this morning in relation to the Platts price-assessment process".

The EC said the big oil companies may have "prevented others from participating in the price assessment process, with a view to distorting published prices".

"Any such behaviour, if established, may amount to violations of European antitrust rules that prohibit cartels and restrictive business practices and abuses of a dominant market position.

It warned: "Even small distortions of assessed prices may have a huge impact on the prices of crude oil, refined oil products and biofuels purchases and sales, potentially harming final consumers."

Luke Bosdet of the AA said British motorists would be relieved that the "lid is finally being lifted off the dark and murky world of oil pricing".

"Because prices are set in secret, drivers and consumers have no idea whether or not the price they pay at the pumps is a fair reflection of the wholesale price."

Shadow energy and climate change secretary Caroline Flint said: "These are very concerning reports, which if true, suggest shocking behaviour in the oil market that should be dealt with strongly.

"When the allegations of price fixing in the gas market were made, Labour warned that opaque over-the-counter deals and relying on price reporting agencies left the market vulnerable to abuse.

"These latest allegations of price fixing in the oil market raise very similar questions. Consumers need to know that the prices they pay for their energy or petrol are fair, transparent and not being manipulated by traders."

Shadow financial secretary to the Treasury Chris Leslie said: "If oil price fixing has taken place it would be a shocking scandal for our financial markets.

"Labour tabled amendments in Parliament last year calling for commodities like oil to be part of the Financial Conduct Authority's regulatory net, but Ministers refused to act. They should explain why they complacently failed to do so."

The raids come six months after the Guardian revealed claims of a gas trading scam that led to investigations by the energy regulator, Ofgem, and the Financial Services Authority (now the Financial Conduct Authority) which is ongoing.

The inquiry by Brussels also comes at a time when the price-reporting agencies are under wider scrutiny and have been told by Iosco, the umbrella group of financial regulators, to tighten up the way they work.

There has been deep unease since the Libor scandal that traded commodities such as oil and gas have become increasingly important as investments and yet many of the transactions are not going through exchanges where prices can be checked and transparency for investors assured.

Mounting concern that energy trading had become an area of potential market abuse was highlighted in a feature in the Financial Times last week. This triggered a response from Platts.

In a letter to the FT this week, Larry Neal, the president of Platts, said: "Your comparison of PRA activity to Libor is a false one … While PRAs do obtain information from 'traders who may have a vested interest in moving the markets, the agencies do not have any such vested interest. In contrast, our role is providing market transparency."

Last week the Guardian reported that some major energy companies, plus banks and trading houses have stopped providing information to the PRAs whose indices have underpinned the wholesale and in turn the retail gas market.

Officials at Statoil, were among those who said that they had ceased co-operating with three PRAs – Platts, Argus and Icis Heren.

Before Christmas, the French oil group Total said in a letter to Iosco: "Sometimes the criteria imposed by PRAs do not assure an accurate representation of the market and consequently deform the real price levels paid at every level of the price chain, including by the consumer."