The salt intake in some bread is so high it is killing 7,000 people a year, campaigners say.
Salt intake in bread is falling, say bakers
Consensus Action on Salt and Health (CASH) said more than a third of the 138 wrapped loaves it checked had salt content above the recommended levels.
The group said if the bread with the highest salt content cut levels to that of the lowest, thousands of lives would be saved each year.
The Federation of Bakers said salt was being cut, but it was a challenge.
The Food Standards Agency's target salt level for bread is 1.1g per 100g.
The lowest salt bread found was Burgen's Wholegrain and Cranberry, with 0.55g salt per 100g, closely followed by Marks and Spencer's Seeded White - 0.6g salt per 100g.
The highest place was shared by two products: Asda's Medium White Big Loaf and Morrison's The Best Farmhouse Malted Bread, which both contained 1.5g salt per 100g.
CASH said if someone ate two slices of toast for breakfast and a sandwich for lunch using Morrison's The Best Farmhouse Malted Bread, then their daily intake from those four slices of bread, at 0.7g of salt per slice, would be 2.8g of salt.
With Asda's Malted Brown Sliced Bread, which contains 0.4g salt/slice, four slices would provide a person with 1.6g salt.
This is a difference of 1.2g salt a day, a fifth of the maximum 6g limit for the whole day. In children the differences were even more marked.
In total, 50 of the loaves surveyed - 36% - contained more than the recommended amount.
In many cases, brand names had higher levels of salt than the supermarket own-label equivalent.
Some 64% of the breads that did not meet the target were branded products.
In the case of Warburton's, the UK's second-largest grocery brand, 15 out of 18 of its breads that were surveyed had a salt content higher than the FSA target, whereas all of the Sainsbury's and Waitrose breads surveyed were below the FSA target.
CASH chairman Professor Graham MacGregor said a cut in the salt content of bread would help tackle high blood pressure and therefore heart attacks and strokes.
"If bakers cut their salt levels to those of the lowest salt producers already on the market, they would effectively save the lives of 7,000 people each year in this country," he said.
"The public should boycott these higher salt breads until they are reduced."
Gordon Polson, director of the Federation of Bakers, said the industry had been making improvements in recent years - salt content had been reduced by 10% since 2005.
But he added: "The reduction of salt in bread thus far has been immensely challenging for the entire industry as salt plays such a critical role in dough formation.
"It is widely recognised that any change that may diminish the flavour would be counterproductive to the objective of improving diets as it is acknowledged that bread plays an important role in a healthy and balanced diet."