National Grid has issued a notice to the energy industry asking for more generation to be brought on to the system following "multiple plant breakdowns".
The power firm said the move was not an indication that there was an immediate risk of blackouts, but it is the first time such a notice has been issued for three years.
An additional 500 megawatts is being requested for later today - equivalent to the power surge experienced at half time during a big sports event.
A statement said: "National Grid has issued a notice to the industry today asking for more generation to be brought on to the system.
"This is part of our standard toolkit for balancing supply and demand and is not an indication there is an immediate risk of disruption to supply or blackouts. It indicates that we would like our power held in reserve to be higher.
"This Notification of Inadequate System Margin (NISM) is a result of multiple plant breakdowns. An additional 500 MWs is being requested for between 16.30 and 18.30 this evening."