The Competition Commission is to get tough with suppliers because its investigation into the existence or otherwise of the 'waterbed effect' has been hampered by a poor response.

As part of its grocery inquiry, the Commission is attempting to establish whether large multiples are charged lower prices for goods at the expense of smaller retailers and wholesalers.

But so far only 15 of the 40 major manufacturers contacted by the Commission in October have given details of the prices they have charged different customers for their top six SKUs, by value, over the past five years.

The Commission said it was determined to gather more data and had reminded suppliers dragging their feet that it could use its statutory powers to force responses.

"We are trying to make sure we have sufficient and representative information from the whole supply chain," said chairman Peter Freeman. "We're not at the stage of using our statutory powers, but it's always there as a threat."

Preliminary analysis of price data from the 15 suppliers does not indicate any waterbed effect on price, according to the Commission's Emerging Thinking document.

But it did say that a waterbed affect could be observed in non-price related factors, such as security of supply, range and quantity, and that the effects of this on competition would be investigated. The FWD said it was concerned that the investigation would not be sufficiently comprehensive because of a lack of response from suppliers.

"We urged the Commission to delve deeply into major suppliers' price records, especially for larger quantities, so it would be able to see the true state of affairs regarding supermarkets' buyer power once and for all," said John Murphy, director general. "The Commission makes several mentions in the Emerging Thinking of its need for more information from suppliers. We agree this is neccessary."

The ACS has submitted an economic model setting out how the waterbed effect may lead to higher prices on average for customers. "We are in dialogue with the ACS," said Freeman.