Food watchdogs yesterday urged parents to pay renewed attention to the labels on certain products for their children after a survey revealed that some crisps, ready meals and cereals are giving them twice as much salt and fat as healthier versions. The Trading Standards Institute found it was still possible to buy popular brands that would give children nearly twice as much fat in a day as is recommended and well over twice the maximum recommended amount of salt.

Trading standards officers bought a range of breakfast cereals, cereal bars, crisps, lunchbox products, sweets, ready meals, pizzas, tinned products such as beans, desserts and drinks and compared those with the highest levels of salt and fat with those with the lowest.

The best cereals contained only 0.1g of fat per serving, compared with the least healthy, which had 0.5g. The fattiest cereal bars had 18.2g of fat compared with 0.5g in the least fatty. While some ready meals for children had only trace amounts of salt, some had 6.9g, when the maximum recommended daily salt intake for a child aged four to six years is 3g.

Phil Thomas, food spokesman for the TSI, said: "It's obvious from the variations that we found that some manufacturers are able to make similar foods with considerably less fat and salt than others."

The TSI has declined to name the brands involved but urged parents to check labels. It is also calling for food manufacturers and supermarkets to use the Food Standards Agency's traffic light system of food labelling, which identifies foods high in salt, fat and sugar.

While some supermarket groups have signed up to the initiative, several leading manufacturers and Tesco have adopted their own different and less judgmental scheme.

Graham MacGregor, chairman of Consensus Action on Salt and Health (Cash) and an expert in cardiovascular medicine, said it was shocking that food manufacturers were still producing foods aimed at children that contained large amounts of salt, despite the clear evidence that it led to high blood pressure and was a major cause of strokes and heart disease and a risk factor in osteoporosis, cancer and asthma later in life. Cash singled out Scooby Doo chicken burgers for containing almost 3g of salt, one and a half times the daily limit for a three year old, Nestlé's Golden Grahams and Cheerios for having 2.5g and 1.25g of salt per 100g, and Kraft Dairylea Lunchables Stackems for having 2g of salt per serving.

A bowl of Golden Grahams, a brand aimed at children, had the same salt content weight for weight as seawater, Professor MacGregor said.

Kraft has said its Lunchables brand will "change beyond recognition" when it is relaunched in the autumn.

Nestlé defended its record, saying that it has been reducing salt in its UK breakfast cereals for the last 10 years. "Since 1998, we have reduced salt by nearly 30% and will continue to reduce these levels without compromising taste. Furthermore, 25% of our brands contain no added salt," a statement said.

Nestlé also argued that Cash's figures on Golden Grahams were based on a 40g portion, whereas it recommends a 30g serving for a child.