The groceries inquiry took an explosive turn this week as the Competition Commission wielded its statutory powers in a bid to uncover the identities of suppliers mistreated by the big four and establish whether the 'waterbed effect' exists.

The Commission has slapped Section 109 notices on "a small number of trade associations" - known to include the FDF and Dairy UK - insisting they hand over papers relating to complaints by members that would fall under the supermarkets code of practice.

The Commission said it had also issued a Section 109 notice to one supplier and final notices to others, indicating more Section 109s were imminent.

Failure to comply with a Section 109 notice, issued under the Enterprise Act 2002, can lead to a fixed fine of up to £30,000, or a daily penalty of up to £15,000 a day. It is believed the notices and warnings sent to individual companies related to efforts to find out whether suppliers squeezed by the big supermarkets make up their margins by charging wholesalers more - leading to the so-called waterbed effect.

In January's Emerging Thinking, the Commission said it had not found any evidence of this - but also conceded it had received just 15 responses to a questionnaire sent out to 40 suppliers asking for details of payment terms. In March, the Commission pledged to "ensure" more manufacturers supplied information related to this issue.

That the Commission is being true to its word will delight wholesalers and independents dismayed at its earlier conclusion the waterbed didn't exist.

Meanwhile, the decision to tighten the vice on trade bodies raises the prospect that, for the first time, full details of complaints of retailer abuse lodged by suppliers with their industry associations will be revealed to the supermarkets themselves.

Applying only to Tesco, Asda, Sainsbury's and Morrisons, the code, which came into force in March 2002, outlaws several supply chain practices, such as applying retrospective discounts at short notice.

It has proved toothless. Manufacturers have been unwilling to go public with their grievances for fear of being delisted - but allegations of dubious practices by the big four remain rife.

The Commission, in Emerging Thinking, urged wronged suppliers, in vain, to come forward, saying: "We are confident we will be able to meet any concerns suppliers may have in relation to confidentiality." This week it insisted: "These notices do not change our approach to confidentiality. We will continue to protect the identity of companies and individuals if they wish and they will be fully consulted before we approach the parties with any questions that might relate to their cases."

But in reality it is likely the Commission will have to tell supermarkets full details of complaints - including the complainant's identity - to allow them to defend themselves.

On the one hand, this will make suppliers uncomfortable - but on the other, they will also be able claim in mitigation that the revelation of the complaint to the Commission was against their wishes.

One industry source said: "It's a good wheeze because it lets suppliers off the hook."