Due to be built over the next 10 years at Hinkley, close to Bristol and roughly 240km across the channel from Wexford, the deal is expected to clear the way for a spate of new reactors across Europe.
The Greens in Ireland accused European chiefs of breaking their own competition rules and the Irish Government of turning a blind eye.
Party leader Eamon Ryan said the European Commission sign-off allows electricity to be generated at Hinkley at twice the price of alternative renewables.
“The decision supports an uncommon market, which brings us back to a highly centralised energy model and hinders the development of a cleaner, safer and more efficient energy future for everyone,” he said.
Greenpeace decried the decision on safety grounds while Austria, in a letter to the Commission, said it would reserve its right to challenge it in the European Court of Justice due to the subsidy element.
The Department of the Environment here, however, said a study by the Office of Radiological Protection found the plant would have no measurable radiological effect on Ireland, or on the Irish Sea.
Ireland can be expected to take some of its electricity needs from the new plant, through the state-owned EirGrid East-West two-way inter-connector from Meath to Wales.
Michael Moynihan, Fianna Fáil spokesman on natural resources, called on Taoiseach Enda Kenny to state if he made any representations to British prime minister David Cameron.
Competition commissioner Joaquin Almunia said: “The authorities have demonstrated that the construction could not be done by market forces alone. This is a market failure and is due to the unprecedented nature and scale of the project.”
As a result, they will guarantee the £17bn (€21.6bn) debt the operator will raise on the markets and will eventually have a capital of about €43bn.
They will also guarantee a minimum price of £92.50 (€117) per megawatt-hour for 35 years for the electricity produced by the plant, which is close to twice the current cost of electricity.
The Commission has said as soon as the operator reaches a 13.5% profit, 60% of the additional amount goes back to the state. The decision will also open the way to build a similarly large nuclear station in Sellafield.