Frustrated nutritionists from leading food manufacturers and caterers have set up a new group to urge the School Food Trust (SFT) to "take a more pragmatic approach" to meals provision.
Rachel Brown, company nutritionist at school meals supplier Green Gourmet, said members were concerned about how nutrient-based standards would be applied in practice.
Many manufacturers felt the standards, which will apply to primary schools this autumn and secondary schools next year, were "overly complicated and probably unnecessary", she said.
"There is also a lack of clarity on the definition of fibre, meat and sugar, where little things make a big difference, such as whether we measure non-milk extrinsic sugars [as laid out in the new standards] or total sugars - which we would prefer to use.
"The definition of meat and meat products is also causing confusion, while we would prefer fibre to be defined according to the AOAC method of analysis [instead of the NSP approach proposed by the SFT]. We just feel that people like us, who are at the sharp end of all this, weren't consulted properly when any of these standards were developed."
Other members are worried about how to get minerals like iron and zinc into menus within the food-based rules, said Gaynor Bussell, nutrition manager at the Food and Drink Federation.
Such concerns were echoed by the Local Authority Caterers' Association (LACA), which claims that a draconian approach to school food provision has caused uptake to decline and driven some caterers to the brink of collapse.
It said: "A lot of caterers are already struggling implementing the food standards, never mind nutrient-based ones. Everyone will interpret them differently, because they are so complicated, and people don't have the time or the expertise to get it right. There is also a feeling that Ofsted inspectors don't have the skills to police them properly anyway."
Last year's ban on all fizzy drinks and bagged snacks had further exacerbated financial problems for schools, said LACA.
However, the SFT insisted that the nutrient standards, which will require menu planners to use software to analyse the nutrient content of meals, were not a step too far: "We've given plenty of advice on our website about the pros and cons of the relevant software."