For many, it's as essential to a meal as knives and forks. A die-hard few won't even taste their food without it. But most of us know by now that we should go easy on salt if we want to avoid high blood pressure.

Unfortunately, that's easier said than done. It emerged this week that pre-made sandwiches can contain more than half the recommended daily allowance of salt.

Ready-made sandwiches are the fuel of the British economy, powering most of us through our working lives, so the discovery that some are loaded with salt should make us sit up and take notice.

Of 140 sandwiches tested by the health lobby group, Consensus Action on Salt and Health (Cash), 40% had 2g or more of salt, or one-third of an adult's recommended daily allowance.

The saltiest sandwich was Asda's Extra Special Yorkshire Ham and Hawkes Wensleydale, which was found to contain 3.9g, or 65% of the recommended daily salt intake.

Pret a Manger's All-Day Breakfast, with 3.5g, was next on the list, followed by Tesco's Finest All-Day Breakfast, Morrison's Chicken and Bacon, Sainsbury's Taste the Difference Brunch and Waitrose's Sausage, Egg and Bacon, all with 3.5g of salt.

Cash noted that a standard bag of Walkers ready-salted crisps contains 0.5g of salt, so the saltiest sandwiches contain the equivalent of seven bags of crisps.

The sandwiches with the least salt were Co-op's Healthy-Living Tuna and Cucumber and Tesco's Healthy-Living Chicken Salad, both with 0.6g per serving.

Jim Winship, the director of the British Sandwich Association, said the criticisms were a "nonsense" as 140 types of sandwich were not representative of such a huge industry. He also stressed that he did not know of a single sandwich maker in the UK that adds salt to sandwiches. However, he added that the industry was working with the Food Standards Agency to reduce the amount of salt in its products.

Why salt content should matter is simple. The body needs salt (sodium chloride) to maintain the concentration of body fluids at correct levels. It also plays a central role in the transmission of electrical impulses in the nerves and helps cells take up nutrients. But too much raises blood pressure, increasing the risk of heart attack and stroke, two of the three biggest killers in the UK. "Around 40% of strokes could be prevented through the control of high blood pressure - that's around 50,000 a year," said Dr Isabel Lee, research liaison officer for the Stroke Association.

The fact is that most of us are eating too much salt. The Food Standards Agency recommends a daily intake for adults of no more than 6g, but currently the average level is about 9g.

There are signs that the message is getting through. In health-conscious circles, adding salt to food is becoming almost as socially unacceptable as smoking. But salt sprinkled on food accounts for only 10%-15% of dietary salt. An estimated 75% is already present in bought food.

This puts the onus on manufacturers to reduce salt levels. Last year, the Food Standards Agency published guidelines for manufacturers on salt reductions in 85 foods to be introduced by 2010. Crisps, for example, should have 1.5g of salt per 100g - a reduction of 10%. But Cash accused the FSA of bowing to industry pressure by increasing its target salt levels in some foods.

So consumers must be vigilant. It's the sodium in salt that can be bad for your health. Sodium is usually listed on labels, salt less often: to get the salt value, multiply the sodium level by 2.5. "High" is more than 1.5g of salt per 100g.

For consumers to make truly informed choices, Dr Lee believes a "mandatory and consistent" scheme allowing people to make like-for-like comparisons is required.

In the meantime, put the salt cellar at the back of the cupboard, choose reduced-salt foods and go easy on sauces such as ketchup and soy.