EDF Energy has said it will stop using coal-fired plants in order to cut carbon dioxide (CO2) emissions from its electricity generation by 60% by 2020.
Power firms in Europe are under pressure to cut their emissions
Instead the firm, the UK arm of the French group, will rely largely on two gas stations. Renewable and nuclear energy will supply the rest equally.
But emissions will rise for a "limited period" as the firm makes the shift and before old power plants are closed.
Pressure on firms to cut emissions, a factor in global warming, is growing.
According to EDF Energy spokesperson Gareth Wynn the cut "is far in excess of any regulatory demands" but the company sees a "competitive advantage" in making the move.
The EU has provisionally set a target to cut greenhouse gas emissions, including carbon dioxide, by 20% - based on 1990 levels - by 2020.
For a period EDF Energy will use both the old and newer energy sources - during which time "total carbon emissions will increase as more electricity is produced". It did not reveal a time-frame for this increase.
But the firm said the UK would benefit because the new plants "would displace power produced by firms using less carbon-efficient means".
The planned 60% reduction is equivalent to taking 3 million cars off the road, EDF Energy said, and comes as part of wider plans to reduce the company's impact on the environment.
Some 60% of EDF Energy's electricity will come from gas power stations.
Renewable energy will supply around 20% of total output and - provided the UK nuclear sector is allowed to expand - it will build one nuclear power station to meet the remaining 20%.
In addition to the 60% cut, EDF Energy said it would reduce CO2 emissions from consumers' energy use by 15% by 2020 and encourage efficiency, for example by promoting smart meters.
The European firm - which supplies around 5 million UK customers - also announced plans to improve the efficiency of its offices, by reducing transport use, and halving waste sent to landfill by 2012.
Reducing its waste will be achieved by cutting the use of raw materials - especially coal, but also paper and copper used for cables.
Moves by EDF Energy to make its power stations more efficient and cleaner started in 2006, when it embarked on investing £40m to install new high-pressure steam engines.