There is concern that the Competition Commission's efforts to establish whether the waterbed effect exists will not go far enough.

Wholesalers' and independents' leaders claim supermarkets use their buying power to squeeze suppliers, who in turn claw back their margins from smaller customers through higher prices.

But the Commission said in its Emerging Thinking that "a preliminary analysis of price data for 15 suppliers of major branded goods does not indicate the presence of price differentials that are consistently in favour of larger retailers".

This has dismayed the FWD. In a letter to the Commission, director general John Murphy wrote: "We feel that the tenor of Emerging Thinking tended to sound somewhat conclusive. We find it extraordinary the Commission should come forward at this point with such a conclusion based on 'a preliminary analysis' of 'detailed pricing evidence' from just 15 suppliers.

"This begs three questions. Were the 15 suppliers representative across all the major product categories? Did they provide prices for like-for-like large volume purchases? Are these price comparisons taking into account every element of the price equation? The Commission says it will 'seek to review additional data for other suppliers'.This is a critical must-do."

A Commission spokesman insisted no conclusions had been drawn. "We're aware the data was not comprehensive. We haven't been conclusive on anything."