A healthy diet can be up to 57 per cent more expensive in the average convenience store than in some large low-cost retail outlets.
A study by the Irish Nutrition and Dietetic Institute found the cost of special diets and the requirements for a typical healthy diet can vary considerably, depending on where a person shops.
It concluded that it is often impossible to find all the ingredients to consume a healthy diet in convenience stores.
"The range of brands and types of products is often rather limited, often without fresh meat or vegetables being available," it said.
The study, commissioned by the Department of Social and Family Affairs, also examined the cost and availability of special prescribed diets such as gluten free or liquidised altered-consistency diets.
Currently, people who have been prescribed a special diet but cannot afford the ingredients within their normal weekly income can apply for supplementary welfare payments from the Health Service Executive.
The test of affordability is whether the cost of the special diet is more than one-third of their income.
The report found that a person on the lowest social welfare income could meet the full cost of the most expensive prescribed special diet for less than one third of their weekly income if they shopped at one of the large low-cost stores.
It also found the average additional cost of a special diet was between €3 and €8 per week, depending on the type of diet concerned and on the type of shop or supermarket in which people normally buy their groceries.
Minister for Social and Family Affairs Seamus Brennan said he recognised that not everybody is in a position to do their shopping at the cheapest shops.
He said he intended to base the new diet-supplement scheme on the food costs at convenience stores to protect the position of those who cannot shop around because of location, age or for whatever reason.
Mr Brennan said this research had provided a solid up-to-date basis for calculating the level of diet supplement payment needed to achieve his view that nobody should have to spend more than one-third of their income on their prescribed diet.
Fine Gael's social and family affairs spokesman David Stanton criticised Mr Brennan saying he was in possession of the study for nine months but did not act on it. He said the inaction was "especially grievous" considering the Diet Supplement Scheme was curtailed as part of his predecessor Mary Coughlan's so-called "Savage 16" social welfare cutbacks two years' ago.
Some 2,000 people, who require special diets for medical reasons, were unable to receive financial assistance after the curtailment of the scheme.
"The Minister was aware of the hardship many people had to endure as a result of this cutback yet in Budget 2006 he did not reverse or amend it."
