Sizewell B provides about 3% of the UK's electricity
Hundreds of thousands of homes suffered power cuts after a fault caused an unplanned shutdown at the Sizewell B nuclear power plant in Suffolk.
Homes and businesses in London, East Anglia, Cheshire and Merseyside were affected between 1100 BST and 1300 BST.
The shutdown cut off supplies to the National Grid within minutes of another plant - the coal-fired Longannet power station in Fife - going offline.
A National Grid spokesman said normal service was expected on Wednesday.
He said: "We think it will be a case of business as usual again tomorrow. We have a very robust system in the UK, it rarely fails and that's why everybody is talking about it so much."
A British Energy spokesman said the Sizewell B shutdown was caused by a fault with conventional equipment and not any part of the nuclear reactor.
It is the first unplanned shutdown at Sizewell for more than three years.
British Energy said there was no safety issue but could not confirm when the problem would be fixed.
Sizewell provides around 3% of the UK's electricity.
Meanwhile, independent energy consultancy McKinnon & Clarke called on the government to build new power stations or face further power cuts.
Energy analyst David Hunter said: "The government's inability to make long-term energy security decisions over the last decade is coming home to roost.
"Since the 'dash for gas' in the 1990s, the lack of political will to make tough decisions has left Britain short of power."