Aldi and Lidl may be tough negotiators, but they are rarely guilty of the kind of “forensic” auditing and unfair charges highlighted this week by the Groceries Code Adjudicator, say suppliers.

Speaking at a Westminster Forum conference, Christine Tacon said the biggest complaint from suppliers was retailers’ use of third party auditors to trawl through records going back as far as six years to find reasons why suppliers owed them money.

She also raised concerns about high charges for customer complaints, suppliers paying for inaccurate retailer forecasting, retailer demands for suppliers to use preferred and more expensive partners, and fines for items that go missing between the supplier and the retailer.

In the wake of her comments, suppliers said the discounters were not big offenders on these counts. “Aldi and Lidl are simpler to deal with and do not have the scale of additional charges that the top four have,” said one supplier.

Another said: “Their ‘keep it simple’ business model does not allow for the complexity which arises once you start changing agreements unilaterally.”

At the conference, Tacon said she had received far more complaints about some retailers than others.She also said she planned to change the funding arrangements for the £800,000 a year cost of running her office so that the retailers causing the most trouble would pay the most. Currently, the 10 grocery retailers affected by the code pay 10% each. “There will be a strong indication of which retailers are causing me problems,” she said.