A supermarket is challenging a ban on advertising its whole-fat milk during children's television shows.
Asda can only advertise its skimmed milks to children
Asda's whole milk narrowly failed to meet new Food Standards Agency (FSA) guidelines which allow only healthy foods to be advertised to children.
Whole-fat milk should be nutritious enough to pass tests based on official figures, but Asda's failed.
The supermarket chain has written to media watchdog Ofcom demanding changes to the testing formula.
Ofcom brought in the new guidelines to help tackle rising obesity levels among children.
It asked the Food Standards Agency (FSA) to determine what constitutes healthy and unhealthy food.
The new rules, introduced in April, mean adverts for food and drinks high in fat, salt or sugar cannot be broadcast around shows aimed at four to nine-year-olds.
An Ofcom spokeswoman said the FSA would review its nutrient profiling model after a year of use.
"Ofcom, as the regulator, doesn't have specific expertise in this area and so relies on the FSA and the experts that they brought in to develop this scheme," she said.
The FSA said whole milk could be advertised to children, based on official UK food tables regarding its nutrient content.
"It appears that on the basis of the calculations that Asda have provided, their whole milk differs from the figures in the official UK food tables," an FSA spokeswoman said.
"It is up to Asda to determine why their milk differs and of course they are welcome to talk to us about it."
But a spokesman for Asda denied the claim, saying: "The suggestion that our milk is different from anyone else's is ridiculous. We source our milk from the same dairies as the other UK supermarkets.
"The issue here is it appears the FSA's nutritional information for milk is not accurate.
"We have run the FSA's profiling model on all other supermarket milk and none of it would, by the FSA's definition, have been able to be advertised to kids."
Dr Judith Bryans, director of the Dairy Council which represents milk producers and processors but not retailers, said there was confusion over how the FSA's formula should be applied.
Dr Bryans said: "The guidance needs to be very, very clear and I don't think it is just at the moment. I think it is detrimental to everybody, this confusion."
The communications regulator uses the FSA's nutrient profiling model to decide which products high in fat, salt or sugar should be subject to the restrictions.
In a letter to Ofcom chief executive Ed Richards and Culture Secretary Tessa Jowell, Asda describes the rules as a "serious cause for concern".