Ready meals and breakfast cereals often contain far more fat and salt than claimed on their packaging, according to new research that may make shoppers think twice about eating convenience food.
Laboratory tests found manufacturers of processed food often misled consumers, with some products having up to 91 per cent more fat than was stated on the label.
Unrealistically small portion sizes encouraged people to underestimate the calories they were consuming, while some "healthy" supermarket products had more salt or sugar than economy ranges.
An investigation by Channel 4's Dispatches programme, which goes out tomorrow night, looks at Britain's 70bn-a-year food industry.
According to figures it obtained from the Government, the British have the worst diet in Europe and each year around 70,000 people die prematurely.
Dispatches established that there is no legal stipulation on the accuracy of labels stating levels of fat, sugar and salt.
Instead, guidelines allow manufacturers a margin of error of up to 30 per cent on fat or salt content.
However, laboratory analysis showed some products substantially exceeded even these margins. Six samples of a Waitrose chocolate pudding contained an average of 45 per cent more fat than was stated on the label, with one sample exceeding the amount quoted on the label by 64 per cent.
Two out of six samples of Sainsbury's chicken curry ready meals were much fattier than shoppers were led to believe. One had a third more fat, whilst another had 91 per cent more fat.
A fifth of 43 products exceeded the margin for error on fat content.
Alan Richards, president of the Association of Public Analysts, called for the Government to reduce the margin for error to 10 per cent, saying: "The consumer is entitled to believe what's written on the labels, and I think supermarkets have a responsibility to ensure these labels are as accurate as possible."
Mr Richards, the head of scientific services for Durham County Council, added that there was also now a "crisis" in public health analysis with only 39 professionals keeping a check on commercial food.
Dispatches found that own-brand ready meals in supermarkets with names such as "Healthy Living" or "Be Good to Yourself" were sometimes less healthy than cheaper, economy versions. In addition, some branded products promoted as healthy had more salt or sugar than junk food.
With 1.55g salt per 100g, Kellogg's All Bran was saltier than a packet of ready salted crisps, while Jordan's Country Crisp muesli had more fat than a pack of cheap sausages. Premium "unadulterated" Dorset cereal had seven times more sugar than an economy muesli.
Tesco and the big food manufacturers have rejected the colour-coded traffic light labelling backed by the Government in favour of a system based on the recommended daily amount of nutrients. Dispatches said there was evidence from the National Heart Forum that the Guideline Daily Amounts were set too high 20 years ago.
In addition, cereal makers were underestimating portion sizes. The programme found that seven-year-olds in Hertfordshire were pouring 43g of cereal into their bowls compared with the 30g quoted on the label.
Philip James, a scientist who helped set up the Food Standards Agency, said: "The consumer cannot understand what's on that label because a lot of it is intrinsic nonsense."