The code, a voluntary agreement by the four largest grocery multiples on trading practices, was drawn up by the Office of Fair Trading after the Competition Commission's monopoly report on the supermarket sector in 2000.
However, in the Remedies Statement issued in the Safeway inquiry, the Competition Commission reported: "Evidence received to date suggests that for the vast majority of suppliers the code of practice had made no difference to their negotiating position with the large supermarkets.
"Of those who thought it had made a difference, more thought it had made the position worse than thought it had made it better."
The operation of the code is currently being reviewed by the OFT, although a spokesman was unable to say when its report might be published.
He said the OFT was still seeking more information about its operation and was investigating whether the code was working and how effective it had been.
Critics of the code point out that only one complaint was lodged in its first year of operation.