Kellogg's Rice Krispies contain 36 more calories per 100g than in 1983 - an increase of about 10% - while Kraft Dairylea Triangles contain 15 more calories per 100g than in 2001, a 7% rise. While cheese used to be their main ingredient, followed by skimmed milk, whey and butter, it now comes third and accounts for just 16%.

Häagen-Dazs Belgian Chocolate ice-cream - always marketed as a dangerous pleasure - contains 16% more calories than in 1994, and 26% more fat. Even products marketed as healthy options are not immune to this "calorie creep". Jordans Original Crunchy bars have 16% more calories than in 1986, and more fat.

Experts said the findings, derived from a comparison of current labels with old ones stored in museum archives, fitted a pattern whereby manufacturers remove salt and some types of fat from food for health reasons, only to replace them with sugar and more fat.

"Reducing salt is an excellent measure, but as a result companies are faced with bland processed food," said Tim Lobstein, the former director of the Food Commission who now heads the child obesity programme at the International Association for the Study of Obesity. "The cheap way of flavouring it up is to sugar it. Fat can also help because it helps your tongue notice the flavours - that's why you butter bread," he said.

A Kraft spokeswoman confirmed this is what had happened in the case of the cheese triangles. "We are trying to balance what consumers say they want in terms of the taste they enjoy, while trying to reduce the salt. But instead there's more butter, so that led to a modest increase in the calorie level," she said. The triangles today have more calcium and added vitamin D, as well as a third less salt.

McVitie's Classic Rich Tea biscuits contain 6% more saturated fat than they did in the mid-1990s - but the company insists this is a change for the better. "Following consumer demand we moved from animal to vegetable oil, and later from hydrogenated vegetable oil to palm oil. I think the palm oil must have a higher amount of saturated fat," a spokeswoman said.

Ingredient labelling became the norm in the late 1970s and nutrition labelling in the mid-1990s, but sometimes the limited information stored in the archives of the Museum of London and Museum of Brands, Packaging and Advertising makes it difficult or impossible to tell what has changed. Kellogg's did not give any details of salt, sugar or fat on its early labels, but since a 100g helping of Rice Krispies today contains only 1g of fat, it seems likely that sugar accounts for the 10% increase in calories. Glucose-fructose syrup has been added as an extra ingredient since the 1980s.

Paxo used to give the nutritional value of its sage and onion stuffing according to its dry weight - which meant there were 360 calories in each 100g. But now it gives the quantities in the cooked product, which has significantly more. The company said the cooking process increases the calories, so for years it gave the public nutritional information on a product they were never likely to eat.

Sainsbury's said that improved testing was the reason the amount of sugar in its sliced bread appeared to have increased by 400% in the past decade, to 4g in each 100g. "The sugar content hasn't changed. The sugar that's found as a part of the carbohydrate in the flour can be better identified now," a spokeswoman said. Other companies also cited more rigorous and sophisticated tests as the reason for discrepancies.

But Professor Erik Millstone, of the science and technology policy research unit at Sussex University, an expert on food additives, criticised the supermarkets and others for selecting information to make foods look healthier.

Sainsbury's Gravy Granules, for example, were listed in 1982 as containing a colour, E150. Today the same additive is listed as caramel. "People will look at this and say it hasn't got any E numbers in it," he said. "It's often misleading to give the chemical names without giving the E numbers as well.

"For me, the form in which nutritional information is presented is singularly unhelpful. I don't actually know how many grams of gravy I eat. If you want to make sense of food labels you have to have a lot of patience, a lot of time and you have to never eat anything outside the home."

Not all the foods examined have changed for the worse. Twiglets have barely altered, while Heinz baked beans, advertised even in 1983 as being free from artificial flavours and preservatives, now contain less sugar, fat and salt. Cadbury's Flake contains less milk and more cocoa than it did in 1983 - the bars are now 25% cocoa and 14% milk solids, while they used to contain 20% of each. Sun-Pat Crunchy peanut butter contains fewer calories and less sugar, though the proportion of saturated fats declared on its label has grown.

But does the increased calorific value of so many shopping trolley staples help explain our thickening waists? In 1997 fewer than 10% of British primary schoolchildren were obese. Now the figure is nearly 17%. And figures from the UN Food and Agriculture Organisation show a striking rise in our calorie intake, to almost 3,500 a day - which means that unless we are exercising more, we should on average be putting on 7kg a year.

Dr Lobstein said the biggest problem remains the fact that we are eating more than we used to. "If you buy a cookie on a station, it's four inches across. Compare that with the biscuits your granny used to eat. Companies are selling to our eyes rather than to our health needs. And it's only if you read the small print that you can tell what's going on."

Nutritional values per 100g/100ml

Heinz baked beans

Sugar

1990s: 6g

2007: 4.9g



Salt (sodium)

1990s: 0.5g

2007: 0.3g



Fat

1990s: 0.3g

2007: 0.2g



Calories (kcal)

1990s: 72

2007: 69



Kraft Dairylea Triangles

Sugar

2001: 5.6g

2007: 5.3g



Salt (sodium)

2001: 1.2g

2007: 0.8g



Fat

2001: 17.5g

2007: 19.5g



Calories (kcal)

2001: 225

2007: 240



Jordans Original Crunchy bars

Sugar

1986: *

2007: 29.3g



Salt (sodium)

1986: *

2007: trace



Fat

1986: 20g

2007: 22.8g



Calories (kcal)

1986: 400

2007: 464



Diet Coke

Sugar

1985: *

2007: 0g



Salt (sodium)

1985: *

2007: trace



Fat

1985: *

2007: 0g



Calories (kcal)

1985: 0.27

2007: 0.6



Häagen-Dazs Belgian Chocolate ice-cream

Sugar

1990s: *

2007: *



Salt (sodium)

1990s: *

2007: *



Fat

1990s: 14.7g

2007: 18.5g



Calories (kcal)

1990s: 246

2007: 286



Kellogg's Rice Krispies

Sugar

1983: *

2007: 10g



Salt (sodium)

1983: *

2007: 0.65g



Fat

1983: *

2007: 1g



Calories (kcal)

1983: 345

2007: 381



*Information not available