Supermarket chiefs could be summoned to Stormont to explain the fuel price differential of up to six pence per litre.
Consumer Council officials told MLAs that in some areas the cost difference is even more than 6p.
Enterprise, trade and investment committee chairman Patsy McGlone said: "It is grossly unfair. Presumably, the product is the same in various locations, so why should the price be different?
"We would like to have them up here to answer questions."
DUP committee member Gordon Dunne said: "It is absolutely ridiculous and wrong."
The North Down MLA said the big-named supermarkets had steered clear of coming to Northern Ireland during the Troubles but are now "reaping the harvest".
"But, of course, we are all hypocritical because we all use them."
His comments came as Consumer Council chief executive Antoinette McKeown said that petrol prices in the province are higher than anywhere else in the UK – and higher for diesel than anywhere in Europe – and motorists are also still paying more for insurance than their UK counterparts.
Speaking as the council launched its latest report on rising food costs in Northern Ireland, Ms McKeown said that almost nine out of 10 consumers have already changed their habits as a result of higher prices.
The Hard To Stomach report shows that families in Northern Ireland have been hardest hit with a steep dive in disposable income, leaving just an average of £82 per week after bills and essentials are paid for – compared to £149 in Britain.
Ms McKeown said almost nine in 10 consumers in the province (87%) are worried about the cost of their food and grocery shopping, and in an online survey the figure rose to 93%.
Council official Aodhan O'Donnell added another concern when he said there were indications that foodstuffs which are put on promotion are often high in sugar, salt and fat.
"Consumers perceive it to be the case that some foods, which are less healthy and cheaper to produce, are increasingly becoming the choice – in inverted commas – of those trying to make food budgets stretch."
The DUP's Paul Frew referred to the on-going investigation into the production and storage of horse meat which has provoked a crisis in consumer confidence.
"I think – not that we have the full story yet – that it may be that supermarkets squeezed the supply chain so that it led to importing cheaper meat just for efficiency, which would worry me greatly," he added.
The Consumer Councils says that:
• After bills and essentials NI families are left with just £82 per week – compared to £149 in Britain
• Most price promotions are on foods high in sugar, salt and added fats
• Nine out of 10 NI consumers are worried about food costs