Competition Commission investigators could seize computer hard drives belonging to supermarket buyers after the probe into the retailers’ dealings with suppliers took a significant turn this week.

Press reports alleged that the commission had unearthed the so-called smoking gun in the investigation – vital evidence of buyers using threatening or aggressive language in emails to growers or farmers.

The emails are thought to have been sent by Tesco and Asda employees in the run-up to this summer’s £520m price war.

Suppliers have long alleged retailers use their strong buying position to demand discounts for produce and threaten delisting if the producer does not then comply.

It was, in part, due to these claims that the current commission investigation was set up in May 2006 after a recommendation by the Office of Fair Trading which said there appeared to be a case to answer.

But until this week, evidence that would support claims of wrongdoing had not been forthcoming.

Now, though, emails written in a manner that contravenes the supermarket code of conduct set up in 2000 are believed to have been found, according to newspaper reports earlier this week.

A commission official told Farmers Weekly that he couldn’t divulge the contents of any email conversations. “I wouldn’t agree that we have found the smoking gun, it is premature to say that,” he said.

But he added that investigators were preparing to carry out an email trawl.

While some reports suggested that could mean processing 11m pieces of information the commission official told FW that the watchdog had a good idea of what they were looking for.

“We will be looking beyond the inboxes of operators. Just because the email has been deleted doesn’t mean it has gone.

“The evidence is stored on their hard drives and we have the techniques that will enable us to retrieve them,” he said.

But Tesco and Asda denied vehemently any wrongdoing. Both have released statements saying that they had nothing to hide and would comply fully with the investigation.

A Tesco spokeswoman said: “We observe the supplier code of practice and expect the commission to conclude that our relations with suppliers are good.”

A statement from Asda read: “We’ve had the request from the commission for emails between suppliers and buyers and, just as we have over the whole investigation, we’re happy to help in any way we can.”

Tesco customers