The National Grid Electricity System Operator (ESO) said if safe margins for operating the power system are reduced, a capacity market notice is issued as an alert to power suppliers which are paid to make sure electricity generation is available when demand is high.

"The ESO is confident that electricity margins are sufficient for this evening. However, a capacity market notice has been triggered by the automated system," it said on Twitter.

(Reporting by Nina Chestney; Editing by Kirsten Donovan)