MPs and lobby groups have renewed calls for the introduction of a watchdog with legal powers to keep big supermarket chains in line.

In a submission to the Competition Commission's groceries inquiry, the Cutting Edge Remedies Group, brought together by Lib Dem Andrew George MP, has called for the Supermarkets Code of Practice to be replaced by an industry-funded adjudicator - dubbed Ofshop.

The group, which also includes the NFU, the Association of Master Bakers, the British Independent Fruit Growers' Association, Friends of the Earth and ActionAid UK, said Ofshop should be given the power to access any relevant information, including details of all trading terms and trade payment practice from retailers and suppliers, to investigate potential areas of abuse.

"This inquiry presents a once-in-a-generation opportunity to create a level playing field for the whole grocery market, all the way through the supply chain," said George. "The code would be a good benchmark for the adjudicator, but the crucial difference would be that Ofshop would proactively investigate the market. It would not have to wait for a complaint, and would have the powers to spot check.

"If the supermarkets are so confident that they have nothing to hide, they should welcome this proposal as it would provide the evidence that they were operating fairly."

Meanwhile, as the commission continued to sift through millions of emails sent between Asda, Tesco and key suppliers, in a bid to find evidence of abuses of power in the run-up to this summer's price war, a competition lawyer warned that Sainsbury's and Morrisons could yet be dragged into the investigation in a similar way.

"If the Commission finds some proof of unfair practice it will lead to further questions," said Michael Hutchings. "It is late in the day but possible the commission will issue section 109 notices to other suppliers and all major retailers."