Customers searching for the healthy options on supermarket shelves may be better-off choosing the cheaper deals, according to a study which has found that own-brand and "value" ranges have the lowest levels of salt.

An analysis of salt levels in breads, cereals, tomato ketchups, sauces and soups found that in every category the best option was a supermarket own-brand, while the worst offenders were more likely to be major brands such as Kellogg's and Baxters.

The study found that of all sugared cornflakes, Kellogg's Cornflakes had the most salt while Tesco Value Cornflakes had the least. At Tesco's online, a 750g packet of Kellogg's costs £2.26 – 30p per 100g – compared with 14p per 100g for the supermarket's value range.

When it came to white bread the highest levels were in Brace's bread followed by Kingsmill Everyday White while the lowest was Sainsbury's "whole and white".

The study was carried out by the campaign group Consensus Action on Salt and Health (Cash) and comes a day before the Food Standards Agency launches the next part of its campaign to reduce salt consumption in the UK. The FSA will warn customers to be aware of the wide variation in salt levels in these types of food.

Katharine Jenner, a nutritionist and Cash campaign manager, warned that branded options were often the worst. "The supermarkets have generally been leading the way in reducing salt, while well-known brands have been dragging their heels," she said.

"Don't think that by buying the more expensive brands you are getting the healthiest products – often it's the cheaper versions that contain less salt." Rosemary Hignett, the FSA's head of nutrition, said it had identified the trend in bread. "It is true that supermarket own-brand loaves of bread are usually lower in salt than the branded loaves and this is good news for consumers, especially in the current economic climate," she said. "The only way of knowing for sure is by checking the labels for salt and comparing brands, and this applies for all foods."

Some supermarkets have started benchmarking their products against other supermarket lines instead of famous brands because of the differences in salt levels. A spokeswoman for Tesco said the chain had been cutting salt levels for four years. "Tesco has removed over 3,000 tonnes of salt from products in the UK since 2005, including 145 tonnes from our ready meals, and this is an ongoing process. The average Tesco ready meal already contains 10% less salt than the 2010 target set by the Food Standards Agency."

Some of the brands highlighted as having the worst levels of salt said they were also making changes. Heinz was rated worst by Cash in the tomato ketchup category with 3.1g per 100g compared with 0.9g per 100g in Asda Smart Price Ketchup. A Heinz spokesman said the recipe for its tomato ketchup had changed, and the new version would be in shops this month. It has reduced salt levels by 27% in line with FSA targets. "We kicked off our salt-reduction work as long ago as 1986, and since 1997 we have dropped the salt by 32% in Heinz Beanz, 39% in Cream of Tomato Soup and 63% in pasta shapes," he said.

Jack Winkler, professor of nutrition policy at London Metropolitan University, said that making a 25% reduction in salt content without alienating customers was a "real technical achievement".

He added: "Supermarkets and their own-label lines tend to be a progressive force on nutrition because if you are a 'brand' and you do something innovative like cut salt it might offend a few and you might lose business. Your company is tied up with one product or a limited number of products so you are extremely conservative about changing anything in case it harms the brand you have spent years building up."