New figures released by the Food Standards Agency (FSA) reveal that one in five people believe that if they eat enough fruit and vegetables it will counteract any harm done by unhealthy foods.
While a majority recognised the importance of eating five portions of fruit and vegetables a day, only half knew that frozen vegetables and dried fruits could be included.
A survey of more than 2,000 adults carried out for the agency shows that government health messages are doing little more than confusing consumers.
A barely credible four out of 10 people surveyed claimed not to know that fat and sugar can be bad for them, and that foods high in fat and sugar should only be eaten occasionally.
Today the FSA launches an "eatwell" plate, which shows the proportions of different food groups that should be eaten in a single meal.
It shows that fruit and vegetables should make up one third of a meal, with another third devoted to starches such as potatoes, rice and pasta.
Milk and dairy should constitute 15 per cent of the plate, meat and fish 12 per cent, while fat and sugar combined should make up just eight per cent of a healthy meal.
Dr Alan Maryon Davis, president of the Faculty of Public Health, said the levels of ignorance demonstrated by the figures were "staggering", given that consistent health advice had been sent out for decades.
The government is drawing up a strategy to tackle obesity, which is expected to predict that if current trends continue, half the population will be classed as obese within 25 years. Already, almost a quarter of adults, and one in five children are grossly overweight.
