High sugar content can be found in unlikely foods such as savoury snacks, campaigners say.
Researchers scoured supermarket shelves
Researchers found meals such as Tesco crispy beef with sweet chilli and Asda's sticky chilli chicken had more sugar content than vanilla ice cream.
They said labelling was confusing as sugar could be classed as dextrose, fructose, glucose and sucrose.
Industry officials said manufacturers were putting more information than ever on products about sugar content.
Which? scoured supermarket shelves looking at the sugar content of food.
Sugar can either be present in food naturally, or added as a sweetener or preservative.
The Food Standards Agency classifies a product as having a high sugar content if it has more than 15g of sugar per 100g or more than 18g if the portion is more than 100g.
Researchers found that Asda sticky chilli chicken contained 19.2g of sugar per 100g and Tesco crispy beef with sweet chilli sauce had 23.1g per 100g.
Weight Watchers oat digestive biscuits contain 20.5g of sugar per 100g - almost 4% more than McVitie's digestives.
Vanilla ice cream contains 17.9g per 100g, Which? said.
Which? said the labelling system needed to be clearer.
Under current labelling regulations, supermarkets are not legally obliged to include nutritional information on a food product unless a specific claim such as "low sugar" is made.
Which? editor Neil Fowler said manufacturers needed to "raise their game"
"It is no wonder if people are baffled about the amount of sugar they're consuming. Although many companies do voluntarily label their products, not all do."
Under EU rules, products have to breakdown sugar content to dextrose, sucrose and so on.
But about half of the food and drink products on sale in the UK contain recommended guideline daily amount, which includes sugar content.
A spokeswoman for the Food and Drink Federation said: "Which? is pushing at an open door. "While labelling regulations require companies to include a precise description of the sugars manufacturers are already making it easier for people to see exactly the amount of sugar in their food by putting guideline daily amount information on the front of their packs."
A spokeswoman for Weight Watchers said: "We do have a slightly higher sugar content, but more importantly less fat, almost half the saturated fat levels, nearly twice the fibre, and less than half the salt of McVitie's digestive biscuits."
And Asda said: "It's than nature of that particular product that this chilli chicken will have sugar in it - after all it is sweet and spicy."
And a spokesman for Tesco said: "We are not pulling the wool over customers' eyes.
"It is obvious from the name of the product - crispy beef with chilli sauce - that it contains sugar."