Farmers leaders' are set to crank up the pressure on the major multiples, promising to find producers that are willing to waive anonymity and speak directly to the Competition Commission.

In its written submission to the grocery inquiry in June, the National Farmers' Union lists 14 examples of what it claims are unfair practices by retailers against farmers and growers, some in breach of the OFT supermarkets code of practice.

But the Commission has been unable to investigate these claims because farmers and growers have proved unwilling to speak out. NFU head of food and farming Kevin Pearce said: "We are really struggling. We said we would supply some people, but farmers don't want to do it because they're scared stiff. If they are currently supplying a supermarket then they just say they are very happy with their relationship."

Pearce said the NFU now had one grower previously supplying a major retailer who was willing to talk to the Commission about how he had been treated.

"He was happy six months ago, but that changed when the retailer brought in a new buyer, who put the contract out to tender. The grower offered better terms, such as a 5% discount on price. But then the buyer showed his offer to two other producers. They both tendered better terms, and the buyer accepted both and split the volumes between them.

"The existing supplier feels he was simply used to drive profit out of the supply chain."