EDF Energy is extending the operational life of two of its UK nuclear power stations by seven years.
Hinkley Point B in Somerset, and Hunterston B in North Ayrshire, are now expected to remain operational until 2023. Both had been due to cease generation in 2016.
Two other nuclear plants, Heysham in Lancashire, and Hartlepool had their life extended by two years in 2010.
EDF also hopes to build a new power station at the Hinkley Point site.
Last month it took the first step towards that goal when its subsidiary NNB Generation Company was granted a nuclear site licence by the Office for Nuclear Regulation.
The licence means the company has developed the required plans, procedures and structures to build a new power station.
However, the government still needs to give the go-ahead before it can be built. A permit is also required from the Environment Agency.
More than 1,500 workers are employed across Hinkley Point B and Hunterston B, which each generate enough electricity for one million homes.
EDF operates eight nuclear power stations across the UK in total.
Its chief executive Vincent de Rivaz said: "This decision will provide low carbon energy to keep the lights on in the UK and it will safeguard jobs at the plants, in the UK nuclear industry and its supply chain.
"It follows a thorough review of safety over the lifetime of each of the plants."
The announcement comes as reports suggest Chancellor George Osborne will use Wednesday's Autumn Statement to propose the building of 30 additional gas power station.
Last month, Japanese firm Hitachi signed a £700m deal giving it rights to build a new generation of nuclear power plants in the UK.
Hitachi is buying Horizon Nuclear Power, which was intending to build reactors on existing sites at Wylfa, Anglesey, and Oldbury, near Bristol.
It is purchasing Horizon from Germany's E.On and RWE, which are withdrawing from the UK nuclear market.