An Irish Government minister has bluntly told the Republic's multiples they have "a duty to explain" why their prices are so much higher than those in Northern Ireland and the UK, when sterling is falling in value against the euro.

Mary Coughlan, Trade and Enterprise Minister, issued her challenge as concern continued to grow over the scale of the shopper exodus to Northern Ireland, where prices on many items are 20% to 30% cheaper. So intense is the traffic that one Asda supermarket in Enniskillen, County Fermanagh, claims to be the company's second most profitable outlet in the UK, with takings of more than £1.8m a week, despite being just 40,000 sq ft in size.

The minister told parliament the VAT cut in the UK could not explain the price difference. "It remains my view," she said, "that where retailers are imposing significant differentials in the price of goods in the Republic compared with those in their retail outlets in Northern Ireland and the UK, they have a duty to explain to their customers and to the Irish economy why this is being done," she said.

Prices were higher because it was more expensive to do business in the Republic, claimed Retail Ireland, representing the multiples. Ms Coughlan responded that she had asked Forfas, a state enterprise agency, to test this claim by examining business costs in both parts of the island, and she expected its report shortly.

Her Cabinet colleague, Finance Minister, Brian Lenihan, has also ordered an inquiry by the Revenue Commissioners and the Central Statistics Office, in a bid to discover how much the shopper exodus is costing the Irish exchequer. He also told parliament the exodus was driven, "not by the VAT reduction, but by sterling's weakness".

Meanwhile, as a result of the cross-border business boom, the Asda outlet in Enniskillen is taking on an extra 120 staff. Store manager Peter Beckett said sales were up 150% on this time last year.