Environmental groups have hit out at plans to extend the working life of one of Scotland's two nuclear power stations by at least seven years.

Hunterston B in Ayrshire, which provides enough power for one million homes, had been due to close in 2016.

But its operator EDF Energy announced that it expects both Hunterston B and Hinkley Point B in Somerset to remain operational until at least 2023 – the year when Scotland's other nuclear power station at Torness in East Lothian is also due to close.

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It means both Scottish plants will be working past the Scottish Government's 2020 target date for having renewable energy providing the equivalent of all Scotland's energy needs.

Hunterston B, which provides 700 jobs, started working in February 1976 and had been scheduled to close last year, but won an extension to 2016.

After extensive safety reviews of its two plants and continuing work with the independent nuclear regulator, EDF decided it did not need to close so soon.

EDF Energy chief executive Vincent de Rivaz – re-opening the visitor centre for Hinkley Point B in Bridgwater, Somerset – 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."

A Scottish Government spokeswoman said ministers were not opposed to the working life of Hunterston B or Torness which employs 550, being extended, subject to a safety case being made.

She said: "We have consistently made it clear that nuclear energy will be phased out in Scotland over time. But we have also consistently made clear that this does not preclude extending the operating life of Scotland's existing nuclear stations to help maintain security of supply over the next decade."

But Dr Dan Barlow, head of policy at WWF Scotland, said: "Given our growth in renewables and huge potential to use energy more efficiently, Scotland does not need to have this 40-year old nuclear station creating yet more radioactive waste to keep the lights on."