The revelation came as the Sunday Telegraph claimed Asda and Tesco were being investigated over accusations that they threatened to no longer use suppliers if they did not drop prices.

A spokesman for the commission said legal notices, known as Section 109s, had been sent to the chains. He added the notices "allow them to investigate more thoroughly" and would not be issued "without reason".

He said: "We are being thorough and using our powers where we can."

But the supermarkets claimed they had nothing to hide. A spokeswoman for Leeds-based Asda said: "We adhere to the code of conduct between suppliers and supermarkets.

"During the period which the commission want to see our correspondence there were about 11 million emails to suppliers. But we are happy to help and we have got absolutely nothing to hide."

A spokeswoman for Tesco said: "The Competition Commission has issued Tesco and other parties with Section 109 notices in order to gather information quickly that might normally be withheld on confidentiality grounds.

"We have nothing to hide and we are doing what we can to assist the commission with this enormous data request. We expect the commission to conclude that at Tesco relationships with suppliers are professional and act to the ultimate benefit of the customer."

The watchdog is investigating the practices of the four big chains -- Tesco, Asda, Sainsbury's and Morrisons -- as part of a two-year inquiry into their dominance of the national market.

The commission launched the investigation in May 2006 and it is not clear when a conclusion will be published.