Northern Ireland's electricity consumers will have paid £1bn more for their electricity than in the rest of the UK by the end of this decade.
Northern Ireland customers overpaying, electricity regulator says
That is according to Energy Regulator Douglas McIldoon who blames poor negotiations during privatisation.
Mr McIldoon said contracts negotiated in the 1990s have been subsidised by the consumer to the tune of hundreds of millions of pounds.
It works out at an average of about £1,600 a household over two decades.
Following privatisation in the 1990s, the new owners of Northern Ireland's power stations agreed to contracts that allowed them to charge over the odds for electricity.
They also allowed them to keep out competition by ensuring that any newcomers must charge the same until the year 2010.
"The effect is that over a decade and a half customers in Northern Ireland will have paid £1bn more than they should have," Mr McIldoon said.
"Certainly more than £1bn more than they would have if they had been anywhere else in the United Kingdom."
When the Republic's electricity board ESB opened their new gas fired plant at Coolkeeragh, near Londonderry, they wanted to sell electricity more cheaply to customers in Northern Ireland.
However, because of the agreements in the 1990s this cannot happen for another five years.
The SDLP's Tommy Gallagher said the situation was a "scandal".
"The government has allowed consumers to be ripped off. These sweetheart deals should be torn up and refunds given to consumers," Mr Gallagher said.
Sinn Fein's Mitchel McLaughlin called for "a prompt response and a
plan of action" from the government.
"It is time for the British government to accept responsibility for this bad deal and come forward with a plan that can create a level playing field," he said.