A nuclear power station originally planned to shut down in 2010 will continue to generate electricity for another two years.
Wylfa power station on Anglesey in North Wales, the last remaining Magnox site still in operation, will now close in September 2014.
The Nuclear Decommissioning Authority said Wylfa will use one reactor, after its other one was shut down in April because of limited fuel stocks.
Brian Burnett, the authority's head of programme for Magnox, said: "Wylfa's continued generation is excellent news and will deliver significant additional revenue to support our clean-up mission."
The additional income from Wylfa and Oldbury in Gloucestershire, which closed in February after generating electricity for four years longer than originally scheduled, is estimated to be worth around £600m.
Stuart Law, Wylfa's site director, said: "It is a credit to the staff who have supported Wylfa throughout its 41 years of generation and I look forward to working with them through the final phase of generation."
Energy Minister Charles Hendry said: "Wylfa has been generating electricity for homes across the country for over four decades now. This move, based on comprehensive safety assessments, is good for our energy security.
"It will enable Wylfa to continue generating for two more years, safeguarding jobs and bringing in additional commercial income, which will help contribute to the costs of our nuclear decommissioning programme management of the nuclear legacy."