National Grid is responsible for ensuring supply and demand are balanced in Britain's energy systems. It forecast tight supply from 1600 GMT on Thursday.

The grid operator has made a few electricity margin notices in recent weeks and typically calls upon suppliers to make extra generation capacity available at short notice to counter a tight system.

In October, the grid operator said it expected the surplus power margin this winter to be lower than in previous years due to generation outages and plant closures but still within acceptable standards to ensure the system runs safely and securely.

(Reporting by Nina Chestney; Editing by Jon Boyle and David Evans)