Fibre-rich bran flakes fortified with essential vitamins could be classed as junk food under Labour's proposed health reforms.
Ministers are considering changes to the UK's Nutrient Profiling Model that would take naturally occurring "free sugars" into account when determining whether products are healthy.
The move could see cereals such as bran flakes reclassified as "less healthy" despite being high in fibre and low in saturated fat.
Food manufacturers have warned the proposals risk undermining years of work to make products healthier and could discourage consumers from choosing fibre-rich foods.
Kellogg's is among the companies to have raised concerns about the reforms.
Bran flakes typically contain glucose derived from natural sources such as corn or wheat starch to improve taste.
Under the proposed changes, that could be enough for the cereal to fall into the category of products deemed "high in fat, salt or sugar".
Industry figures have warned that cereals widely regarded as healthy could face advertising and promotional restrictions despite evidence linking high-fibre diets to lower risks of heart disease and cancer.
The move could see cereals such as bran flakes reclassified as 'less healthy' despite being high in fibre
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Dean O'Brien, managing director of Kellogg's UK, said the proposals "have the potential to unintentionally undo years of work and investment by food companies to encourage healthier choices".
The company says it has spent the past decade improving the nutritional profile of its cereals, reducing sugar by 27 per cent and salt by 22 per cent.
"Under current regulations all of our children's cereal, and four of our five best-sellers, are deemed healthy," Mr O'Brien said.
"However, under the newly proposed plans, cereals such as bran flakes, which are high in fibre and low in saturated fat, would be re-classified as less healthy."
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Kellogg's is among the companies to have raised concerns about the reforms
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Official figures show that 96 per cent of Britons fail to consume the recommended 30g of fibre per day.
Industry sources accused ministers of "moving the goalposts" after food manufacturers reformulated products to comply with existing regulations.
"The ink was barely dry" on previous rules designed to encourage healthier products, one source told The Telegraph.
"But these could suddenly be treated as 'less healthy' again."
Plain yogurts containing real fruit could also fall foul of the proposed rule
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Plain yogurts containing real fruit could also fall foul of the proposed rules, while some pasta sauce manufacturers may be forced to reduce tomato content in order to comply.
Dairy UK has warned that naturally occurring sugars in milk could affect a wide range of dairy products, particularly lactose-free alternatives.
The trade body said the changes risk pushing consumers towards sugar-free soft drinks rather than nutrient-rich dairy products, potentially harming British farmers.
The Government has defended the proposals, arguing that the existing model is based on nutrition guidance that is now more than two decades old.
They said: "The new model shifts the focus from total sugars to free sugars and promotes diets higher in fibre.
"Obesity and poor diets rob children of the best possible start in life and this is about supporting parents to raise the healthiest generation of children by restricting the advertising and promotion of junk food," the spokesman added.
“We are currently consulting on applying the new model to the advertising and promotions restrictions and will consider all feedback.”Our Standards: The GB News Editorial Charter