Not enough is known about salt substitutes to be sure none of them is harmful to health, it is claimed.

The Food Standards Agency's efforts to persuade food manufacturers to cut sodium levels in processed foods has resulted in increased demand for alternatives in a bid to prevent any flavour deterioration.

But salt suppliers said this week they were concerned about the lack of research into the potential impact of these ingredients.

"These salt substitutes have not been subject to any long-term health assessments," said Peter Sherratt, general secretary of the Salt Manufacturers' Association.

"Despite this, they are considered safer than salt - the oldest preservative known to man and a naturally-occurring mineral."

The SMA said one salt replacement - potassium chloride - had been singled out by the European Commission's Scientific Committee for Food as a source of concern.

The association said the committee had warned "its use could result in excessive potassium intakes, at which toxicity might develop in individuals with undetected renal problems".

Other replacement products include ammonium chloride, citrates, formates and glutamates.

One industry source said: "It is our speculation that salt replacements, which have been on the market for such a short time, cannot have been subject to long-term assessments. Each ingredient on its own may be familiar, but what about the effect when combined? Until 20 years ago salt was considered perfectly safe."

Meanwhile, the FSA has unveiled plans for collecting data on salt content from manufacturers and retailers.

The move forms part of its bid to monitor companies' progress on meeting salt-reduction targets .

It intends to ask for volume sales weighted average data for products over a 52-week period. It also wants to know the minimum and maximum levels of salt in products within categories.