Tests on 70 foods carried out by the consumer group Which? revealed biscuits containing more than three times the amount of saturated fat than that claimed on the label, and pizza with more than 80 per cent extra fat.
One of the worst offenders was Rivington's Pink Panther wafers, with two biscuits containing an unmarked extra three grams of saturated fat, almost one sixth of a woman's guideline daily amount. Also on the list was Tesco Kids hot dog pizza, which contained 47 per cent more sugar than the supermarket claimed.
The whole study found that only seven per cent of the 570 nutrients tested were present in exactly the amounts that product labels said, with 17 per cent outside the agreed margin of error set by Lacors, the local authorities' agency responsible for co-ordinating regulation.
In response to the research, Britain's Food Standards Agency (FSA), which is already working on a new system of simple and accurate labelling for consumers, said it was involved in discussions to change EU regulations and that "we expect this to include specific rules on acceptable margins of error".
"It is the manufacturers' and retailers' responsibility to ensure that the products they sell are accurately labelled and do not mislead consumers, and local food law enforcement officers are responsible for checking that they do so. Action can be taken against companies where labels are found to be misleading," said the agency.
Lacors' guidelines allow for the content of main nutrients to be 20 per cent either side of the labelled value, though this rises to 30 per cent for lesser nutrients making up between two and five per cent of a food.
Paul Lenartowicz, an independent public analyst working for local authorities in Manchester, said legislation to set concrete error margins for nutrition labels "would benefit manufacturers as well by letting them know what they have to comply with".
But Lenartowicz, who is heavily involved in food safety analysis, also said that all manufacturers need to test a range of end products to make sure the 'typical' nutritional content is more accurate, even if this means increased costs for those not already doing so.
Meanwhile, Which?'s research is likely to further damage consumer trust of major food producers and retailers, especially coming so soon after widespread food alerts over the illegal dye Sudan 1.
Britain's mainstream press and media has already begun to question the current reliability of food nutrition labels, and Which? editor Malcolm Coles said in the group's eponymous magazine: "How can you trust what you're eating when so many labels fall outside even the fairly generous margins of error allowed?"
However, Britain's Food and Drink Federation (FDF) has insisted that producers were not trying to hoodwink consumers and that monitoring nutrition content was complicated and often produced varying results.
"Nutrients such as fat and sugars come from a number of ingredients within a product, and their levels will vary for a number of reasons, including the variety of ingredient used or the season in which it was grown," it said.
In statements, Tesco said it had made the necessary changes to its pizza and Rivington said that, as a matter of precaution, it had sent its wafer product off for analysis. Both added that their products' content was rigorously tested before hitting shop shelves.