The raids were prompted by British consumers

P&O, Stena and Eurotunnel all had their offices visited by Commission officials.

The UK raids were prompted by letters from British consumers complaining about the excessive cost and similar pricing of cross-channel ferries.

"The purpose of these inspections is to ascertain whether there is evidence of suspected market-sharing agreements and related illegal practices aimed at foreclosing competition," the commission said in a statement.

The material gathered at the raids, which are continuing, will have to be considered before the decision to make a formal investigation takes place.

Commission inspectors are also reported to have visited a number of other ferry companies in Sweden, Denmark and Germany.

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A Commission spokeswoman said: "Surprise inspections are a preliminary step in investigations into suspected cartels.

"The fact that the Commission carries out such inspections does not mean the companies are guilty of anti-competitive behaviour, nor does it prejudge the outcome of the investigation itself."

Swedish firm Stena AB - which owns UK-based Stena Line - said its offices in Kent and Sweden had been visited.

"Stena AB and its subsidiaries are fully cooperating with the authorities to provide the information required and Stena is confident that such cooperation did not and does not take place," the company said.

P&O also said that it was "fully cooperating with the Commission".

A Eurotunnel spokesman said: " We operate in a fiercely competitive marketplace and we are confident that any investigation will show that."