In a market message, National Grid said there is a reduced margin between the hours of 1600-1900 GMT on Wednesday, with a system shortfall of 584 megawatts.

"An electricity margin notice is used to send a signal to the electricity market. It highlights that, in the short-term, we would like a greater safety cushion (margin) between power demand and available supply," National Grid said.

"It does not signal that blackouts are imminent or that there is not enough generation to meet current demand."

A further update will be issued tomorrow.

(Reporting by Nina Chestney; Editing by David Goodman)