British Energy has said that it would extend the lives of Hinkley Point B and Hunterston B nuclear power stations by five years to 2016.
The two sites, which are currently running at around 60% capacity due to boiler issues, began generating power in 1976.
Further studies will be conducted by 2013 regarding the potential for additional life extensions beyond 2016, the company added.
British Energy said the decision was an important move towards supporting the UK's climate change goals and the reduction of CO2 emissions.
Ayrshire-based Hunterston B and Hinkley Point B, near Bridgwater, Somerset, are both capable of supplying power to more than one million homes.
The potential for life extensions at the company's other nuclear power stations will be considered in due course, British Energy said.
It is also working with companies from around the world in a bid to develop plans for new-build nuclear projects, should such expansion be permitted by the Government.
To enable Hinkley Point B and Hunterston B to operate over their extended lives, British Energy expects to spend an additional £90 million on the sites in excess of its current investment programme.
Both sites were impacted by technical problems last year, resulting in the stations operating at around 60% load. Further work will be undertaken during planned outages over the next year aimed at delivering 70% load.
At such a level, British Energy said it believed it would be economically viable to extend the lives of the two stations.