The headquarters of Britain's four biggest supermarket chains have been raided by trading officials in the latest inquiry into alleged price fixing, it emerged yesterday.

The Office of Fair Trading staged the unannounced visits on Thursday last week at the offices of Tesco, Asda, Morrisons and Sainsbury's and retrieved data said to include hundreds of e-mails to and from suppliers, who were also said to have been investigated.

Though the OFT refused to comment on the investigation, said to be one of the biggest in the organisation's history, all four companies confirmed their premises had been visited.

The watchdog is believed to be investigating the prices of health and beauty products, as well as groceries.

Recent research has claimed the cost of a typical basket of basic items has increased 15% in a year, while food prices have been rising worldwide as emerging economies, led by China, have the means to buy more milk, cereals and meat.

While none of the supermarkets would comment in detail, all denied any form of price fixing, while one insider denounced it as a "fishing trip".

News of the inquiry comes days after the OFT announced accusations that the same "big four" supermarket chains had been involved in colluding over the price of cigarettes, along with two major tobacco firms, off sales, petrol stations and convenience stores.

It has targeted the supermarkets as part of a long-running investigation into milk prices.

The supermarkets are also being examined by the Competition Commission, which is to publish the results of its two-year investigation into the grocery market by May 8.

A spokeswoman for Tesco said that it had helped bring the cost of an average family shopping bill down by 30% over the last 10 years.

She added: "We understand that the OFT has asked for information from a wide range of suppliers and retailers. We are working with them to provide what they require."

Sainsbury's, Morrisons and Asda also said that they were happy to comply with the OFT's requests for information.

Tesco, which takes more than £1 of every £8 spent in the high street, unveiled record profits of £2.8bn this month.

It comes less than two weeks after the OFT alleged 112 companies building schools, hospitals and other public buildings had formed cartels to boost contract prices by millions of pounds. In that case, the OFT said over two-thirds of the companies named have admitted involvement, hoping to lessen any fine. The maximum fine that can be imposed by the OFT is 10% of turnover.

Meanwhile, the watchdog won the first round of a major legal battle with the UK banking industry on Thursday which could eventually see overdraft charges of more than £1bn repaid to customers.

A High Court ruling allowed the watchdog to press ahead with an investigation, begun in March 2007, to determine whether banks can legally charge customers who slip into the red without prior agreement.

But the judge stopped short of saying whether charges for unauthorised overdrafts were unfair.

A spokesman for the OFT refused to comment yesterday.