Energy Secretary Ed Davey told a news conference in London it was a "historic" day.
He said the UK was facing a "looming energy crisis" in the next decade thanks to years of neglect and under-investment.
Much of coal and nuclear-generated energy will stop in the coming years.
"We have known for years this is coming, but no-one was willing to take tough decisions."
Mr Davey stressed the project included plans to cover the costs of de-commissioning, with the operators required to pay into a fund from day one.
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Buckingham Palace, London's Tower Bridge, Old Trafford and Big Ben are taking part in the event to raise awareness about climate change.
Widespread frost for most areas as temperatures dip below freezing.
Premier League clubs have agreed to pay all full-time staff the living wageof £9.15 an hour in London or £7.85 an hour outside the capital.