The nutritional information on some food products is "wildly inaccurate", a consumer magazine has warned.
One type of pizza contained 47% more sugar than the label said
Which? looked at 570 nutrients in 70 products and found just 7% exactly matched the quantities on the labels.
Some 17% fell outside the accepted 20% margin of error, including a "kids" pizza with 47% more sugar than stated.
Currently there is no specific law about how accurate the information on food labels should be - they need only show average nutrition values.
These can be worked out in different ways, none of which is 100% reliable.
Rivington's Pink Panther wafers contained nearly three times more saturated fat than stated on the label
Cadbury's Light Trifles contained 23% more fat than stated
Tesco Kids Hot Dog Pizza contained 47% more sugar than claimed
The most accurate method is to analyse the food.
But even this is not foolproof, as batches of the same food can differ nutritionally.
Lacors, the body that advises trading standards officers about enforcing food laws, says that an error margin of 20% either side of the labelled value was acceptable.
This can be as high as 30% where nutrients make up between 2% and 5% of a food.
Which? editor Malcolm Coles said: "Nutrition labels help people compare foods and make healthy choices, but only if they're accurate.
"How can you trust what you're eating when so many labels fall outside even the fairly generous margins of error allowed?"
Action can be taken
In a statement, the Food Standards Agency said: "It is very important that nutrition labels should provide accurate information so that people can trust the foods they buy.
"It is the manufacturers' and retailers' responsibility to ensure that the products they sell are accurately labelled and do not mislead, 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."
Brigid McKevith, a nutrition scientist at the British Nutrition Foundation, said: "There is draft EU legislation which will tighten up nutrition claims and it is likely that the legislation on nutrition labelling will be updated."
A Food and Drink Federation spokesperson said any suggestion that food manufacturers set out to confuse or hoodwink consumers was not true.
"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.
"Because of this, the law allows manufacturers to use average figures to give consumers a good indication of what the 'typical' nutrient content is for each product."
A Tesco spokesperson said it had made the necessary changes to its pizza product.
"We work hard to keep within the acceptable guidelines laid down by trading standards and carry out thousands of our own tests to make sure that we do."
A Cadbury spokesman said the trifle range contained many ingredients, and there would inevitably be variations from one pot to another.
"However, we are continuously reviewing our manufacturing process and are updating our packs to ensure information is as accurate as possible."
Rivington said its product had been sent off for analysis, and declined to comment further.
Which? analysed the energy, protein, carbohydrate, sugars, fat, saturates, fibre and sodium (salt) content of 70 processed foods.